Mastering Top LeetCode Patterns for Coding Interviews: A 2025 Guide and Strategy

So you're prepping for those dreaded technical interviews, huh? Trust me, I've been there - drowning in an ocean of 2000+ LeetCode problems, wondering if I'd ever make it to the other side. But here's the secret I wish someone had told me earlier: It's not about how many problems you solve; it's about recognizing the patterns behind them. It's not about how many problems you solve; it's about recognizing the patterns behind them. patterns After banging my head against the algorithmic wall for months (and nursing the resulting headaches with way too much caffeine 😅), I finally cracked the code. Today, I'm sharing the most effective LeetCode patterns that'll have you solving problems like a pro in 2025. (and nursing the resulting headaches with way too much caffeine 😅) cracked the code effective LeetCode patterns Why Patterns Matter More Than Grinding Random Problems Look, I used to be that person who'd randomly pick LeetCode problems and pray I'd seen enough variations to ace my interviews. Spoiler alert: it didn't work out great. randomly pick LeetCode problems The truth? Technical interviews in 2025 are less about memorizing solutions and more about applying problem-solving frameworks. According to recent studies, candidates who master key patterns perform significantly better than those who just grind through hundreds of random problems. Technical interviews Technical interviews As one senior engineer at a FAANG company told me: "I don't care if you've solved that exact problem before. I want to see if you can recognize the pattern and apply the right approach." "I don't care if you've solved that exact problem before. I want to see if you can recognize the pattern and apply the right approach." "I don't care if you've solved that exact problem before. I want to see if you can recognize the pattern and apply the right approach." So let's get into the patterns that'll give you the highest ROI for your precious study time! ROI 1. Sliding Window: Your BFF for Array and String Problems The sliding window technique is CLUTCH for array/string problems where you need to find a subarray or substring that meets certain conditions. Instead of brute-forcing with nested loops (and making the interviewer cringe at your O(n²) solution), this pattern lets you solve these problems in O(n) time. When to use it: Working with linear data structures like arrays or strings Need to find a subarray/substring that meets a condition Looking for min/max/longest/shortest subarray with specific properties Working with linear data structures like arrays or strings Need to find a subarray/substring that meets a condition Looking for min/max/longest/shortest subarray with specific properties The Technique: We use two pointers (let's call 'em i and j) to create a "window" that can expand or shrink: def sliding_window_example(nums, k): # Dynamic sliding window example - find max sum subarray of size k window_sum = 0 max_sum = 0 start = 0 for end in range(len(nums)): # Expand the window window_sum += nums[end] # When window exceeds size k, shrink from left if end >= k - 1: max_sum = max(max_sum, window_sum) window_sum -= nums[start] # Remove element going out of window start += 1 return max_sum def sliding_window_example(nums, k): # Dynamic sliding window example - find max sum subarray of size k window_sum = 0 max_sum = 0 start = 0 for end in range(len(nums)): # Expand the window window_sum += nums[end] # When window exceeds size k, shrink from left if end >= k - 1: max_sum = max(max_sum, window_sum) window_sum -= nums[start] # Remove element going out of window start += 1 return max_sum There are two flavors of sliding window: Fixed-size window: When the subarray size is fixed (like "find max sum of subarray of size k") Dynamic-size window: When the size changes based on a condition (like "shortest subarray with sum >= target") Fixed-size window: When the subarray size is fixed (like "find max sum of subarray of size k") Fixed-size window: Dynamic-size window: When the size changes based on a condition (like "shortest subarray with sum >= target") Dynamic-size window: Here's how you'd implement a dynamic window for finding the smallest subarray with a sum ≥ target: def smallest_subarray_with_given_sum(nums, target): window_sum = 0 min_length = float('inf') start = 0 for end in range(len(nums)): window_sum += nums[end] # Add the next element to the window # Shrink the window as small as possible while maintaining the condition while window_sum >= target: min_length = min(min_length, end - start + 1) window_sum -= nums[start] start += 1 return min_length if min_length != float('inf') else 0 def smallest_subarray_with_given_sum(nums, target): window_sum = 0 min_length = float('inf') start = 0 for end in range(len(nums)): window_sum += nums[end] # Add the next element to the window # Shrink the window as small as possible while maintaining the condition while window_sum >= target: min_length = min(min_length, end - start + 1) window_sum -= nums[start] start += 1 return min_length if min_length != float('inf') else 0 Honestly, once I got this pattern down, so many "hard" problems suddenly became manageable. It's like having a secret weapon! 🔫 Practice Problems Maximum Sum Subarray of Size K Longest Substring with K Distinct Characters Fruits into Baskets (LeetCode #904) Longest Substring Without Repeating Characters (LeetCode #3) Maximum Sum Subarray of Size K Longest Substring with K Distinct Characters Fruits into Baskets (LeetCode #904) LeetCode #904 Longest Substring Without Repeating Characters (LeetCode #3) LeetCode #3 2. Two Pointers: Double the Fun! Two pointers is another game-changer, especially when working with sorted arrays. This pattern involves using two pointers to iterate through an array and find elements that satisfy certain conditions. When to use it: Working with sorted arrays Need to find pairs that satisfy a condition Problems involving reversing or palindromes Working with sorted arrays Need to find pairs that satisfy a condition Problems involving reversing or palindromes Basic Implementation: def two_sum_sorted(numbers, target): # Two pointers from opposite ends left, right = 0, len(numbers) - 1 while left < right: current_sum = numbers[left] + numbers[right] if current_sum == target: return [left + 1, right + 1] # 1-indexed result for LeetCode elif current_sum < target: left += 1 # Need a larger number else: right -= 1 # Need a smaller number return [-1, -1] # No solution found def two_sum_sorted(numbers, target): # Two pointers from opposite ends left, right = 0, len(numbers) - 1 while left < right: current_sum = numbers[left] + numbers[right] if current_sum == target: return [left + 1, right + 1] # 1-indexed result for LeetCode elif current_sum < target: left += 1 # Need a larger number else: right -= 1 # Need a smaller number return [-1, -1] # No solution found I swear, this technique has saved me in so many interviews. One time I was blanking until I realized "Oh wait, this is just a two pointer problem!" The interviewer's face lit up and I knew I was back in the game. 😎 Practice Problems: Practice Problems: Two Sum II (LeetCode #167) Remove Duplicates (LeetCode #26) Squares of a Sorted Array (LeetCode #977) 3Sum (LeetCode #15) Two Sum II (LeetCode #167) LeetCode #167 Remove Duplicates (LeetCode #26) LeetCode #26 Squares of a Sorted Array (LeetCode #977) LeetCode #977 3Sum (LeetCode #15) LeetCode #15 3. Fast & Slow Pointers: The Tortoise and the Hare This pattern uses two pointers that move at different speeds. Super useful for cycle detection in linked lists or arrays. When to use it: When to use it: Linked list problems, especially cycle detection Finding the middle of a linked list Determining if a number is a happy number Linked list problems, especially cycle detection Finding the middle of a linked list Determining if a number is a happy number def has_cycle(head): if not head or not head.next: return False slow = head fast = head # Fast pointer moves twice as fast as slow pointer while fast and fast.next: slow = slow.next # Move slow pointer by 1 fast = fast.next.next # Move fast pointer by 2 # If there's a cycle, they'll meet if slow == fast: return True # If fast reaches the end, there's no cycle return False def has_cycle(head): if not head or not head.next: return False slow = head fast = head # Fast pointer moves twice as fast as slow pointer while fast and fast.next: slow = slow.next # Move slow pointer by 1 fast = fast.next.next # Move fast pointer by 2 # If there's a cycle, they'll meet if slow == fast: return True # If fast reaches the end, there's no cycle return False When I first encountered this technique, my mind was blown. 🤯 Like, how does moving at different speeds help? But once you see it in action-especially with the Floyd's Cycle Finding Algorithm-it's pure magic. Practice Problems: Practice Problems: Linked List Cycle (LeetCode #141) Middle of the Linked List (LeetCode #876) Palindrome Linked List (LeetCode #234) Happy Number (LeetCode #202) Linked List Cycle (LeetCode #141) LeetCode #141 Middle of the Linked List (LeetCode #876) LeetCode #876 Palindrome Linked List (LeetCode #234) LeetCode #234 Happy Number (LeetCode #202) LeetCode #202 4. Tree and Graph Traversal: DFS & BFS Trees and graphs are EVERYWHERE in technical interviews, especially at companies like Meta and Amazon.Mastering both Depth-First Search (DFS) and Breadth-First Search (BFS) is non-negotiable. When to use DFS: When to use DFS: Finding a path between two nodes Detecting cycles Topological sorting Exploring all possibilities (backtracking) Finding a path between two nodes Detecting cycles Topological sorting Exploring all possibilities (backtracking) DFS Implementation: def dfs(root): if not root: return # Visit the current node print(root.val) # Recursively visit left and right children dfs(root.left) dfs(root.right) # Iterative DFS using a stack def iterative_dfs(root): if not root: return stack = [root] while stack: node = stack.pop() print(node.val) # Push right first so left gets processed first (LIFO) if node.right: stack.append(node.right) if node.left: stack.append(node.left) def dfs(root): if not root: return # Visit the current node print(root.val) # Recursively visit left and right children dfs(root.left) dfs(root.right) # Iterative DFS using a stack def iterative_dfs(root): if not root: return stack = [root] while stack: node = stack.pop() print(node.val) # Push right first so left gets processed first (LIFO) if node.right: stack.append(node.right) if node.left: stack.append(node.left) When to use BFS: When to use BFS: Finding the shortest path Level-order traversal Finding nodes closest to the starting node Finding the shortest path Level-order traversal Finding nodes closest to the starting node BFS Implementation: from collections import deque def bfs(root): if not root: return queue = deque([root]) while queue: node = queue.popleft() print(node.val) if node.left: queue.append(node.left) if node.right: queue.append(node.right) from collections import deque def bfs(root): if not root: return queue = deque([root]) while queue: node = queue.popleft() print(node.val) if node.left: queue.append(node.left) if node.right: queue.append(node.right) The first time I tried implementing these algorithms in an interview, I mixed up my stack and queue operations. Talk about an embarrassing moment! 😳 Pro tip: practice these until they're second nature. Practice Problems: Practice Problems: Binary Tree Level Order Traversal (LeetCode #102) Number of Islands (LeetCode #200) Course Schedule (LeetCode #207) Word Ladder (LeetCode #127) Binary Tree Level Order Traversal (LeetCode #102) LeetCode #102 Number of Islands (LeetCode #200) LeetCode #200 Course Schedule (LeetCode #207) LeetCode #207 Word Ladder (LeetCode #127) LeetCode #127 5. Binary Search: Not Just for Sorted Arrays! 5. Binary Search: Not Just for Sorted Arrays! Binary search is a classic divide-and-conquer algorithm that's often underestimated. It's not just for finding elements in sorted arrays-it can be applied to various problems with a monotonic search space. When to use it: When to use it: Searching in a sorted array Finding a specific value or a range Problems where the solution space can be divided in half Searching in a sorted array Finding a specific value or a range Problems where the solution space can be divided in half def binary_search(nums, target): left, right = 0, len(nums) - 1 while left <= right: mid = left + (right - left) // 2 # Avoid potential overflow if nums[mid] == target: return mid elif nums[mid] < target: left = mid + 1 # Search in the right half else: right = mid - 1 # Search in the left half return -1 # Target not found def binary_search(nums, target): left, right = 0, len(nums) - 1 while left <= right: mid = left + (right - left) // 2 # Avoid potential overflow if nums[mid] == target: return mid elif nums[mid] < target: left = mid + 1 # Search in the right half else: right = mid - 1 # Search in the left half return -1 # Target not found I used to think binary search was trivial until I started seeing all the clever variations in interviews. Now it's one of my go-to techniques! Remember, binary search isn't just about finding an element-it's about eliminating half of the possibilities in each step. Practice Problems: Practice Problems: Search in Rotated Sorted Array (LeetCode #33) Find First and Last Position of Element (LeetCode #34) Median of Two Sorted Arrays (LeetCode #4) Koko Eating Bananas (LeetCode #875) Search in Rotated Sorted Array (LeetCode #33) LeetCode #33 Find First and Last Position of Element (LeetCode #34) LeetCode #34 Median of Two Sorted Arrays (LeetCode #4) LeetCode #4 Koko Eating Bananas (LeetCode #875) LeetCode #875 6. Dynamic Programming: Breaking Problems Down 6. Dynamic Programming: Breaking Problems Down Dynamic programming (DP) is often the most intimidating pattern for candidates, but it's incredibly powerful once you get the hang of it. The key is breaking down problems into smaller, overlapping subproblems. When to use it: When to use it: Optimization problems (max/min/longest/shortest) Counting problems (number of ways to...) Problems with overlapping subproblems and optimal substructure Optimization problems (max/min/longest/shortest) Counting problems (number of ways to...) Problems with overlapping subproblems and optimal substructure def coin_change(coins, amount): # Initialize DP array with amount+1 (represents "infinity") dp = [amount + 1] * (amount + 1) dp[0] = 0 # Base case: 0 coins needed to make amount 0 for coin in coins: for x in range(coin, amount + 1): # Either don't use this coin, or use it and add 1 to solution for amount-coin dp[x] = min(dp[x], dp[x - coin] + 1) return dp[amount] if dp[amount] != amount + 1 else -1 def coin_change(coins, amount): # Initialize DP array with amount+1 (represents "infinity") dp = [amount + 1] * (amount + 1) dp[0] = 0 # Base case: 0 coins needed to make amount 0 for coin in coins: for x in range(coin, amount + 1): # Either don't use this coin, or use it and add 1 to solution for amount-coin dp[x] = min(dp[x], dp[x - coin] + 1) return dp[amount] if dp[amount] != amount + 1 else -1 I'm not gonna lie, DP problems used to make me want to curl up in a ball and cry. 😭 But once I understood the pattern of defining states and transitions, they became much more approachable. Practice Problems: Practice Problems: Coin Change (LeetCode #322) Longest Common Subsequence (LeetCode #1143) Longest Increasing Subsequence (LeetCode #300) House Robber (LeetCode #198) Coin Change (LeetCode #322) LeetCode #322 Longest Common Subsequence (LeetCode #1143) LeetCode #1143 Longest Increasing Subsequence (LeetCode #300) LeetCode #300 House Robber (LeetCode #198) LeetCode #198 7. Backtracking: Exploring All Possibilities Backtracking is essentially recursion with a twist-it explores all possible solutions by building candidates incrementally and abandoning them ("backtracking") once it's clear they won't work. When to use it: When to use it: Combinatorial problems (combinations, permutations) Puzzle solving (sudoku, n-queens) Constraint satisfaction problems Combinatorial problems (combinations, permutations) Puzzle solving (sudoku, n-queens) Constraint satisfaction problems def permute(nums): result = [] def backtrack(current, remaining): # Base case: all numbers used if not remaining: result.append(current[:]) return for i in range(len(remaining)): # Add the number to our current permutation current.append(remaining[i]) # Recursively backtrack with remaining numbers backtrack(current, remaining[:i] + remaining[i+1:]) # Remove the number to try other possibilities current.pop() backtrack([], nums) return result def permute(nums): result = [] def backtrack(current, remaining): # Base case: all numbers used if not remaining: result.append(current[:]) return for i in range(len(remaining)): # Add the number to our current permutation current.append(remaining[i]) # Recursively backtrack with remaining numbers backtrack(current, remaining[:i] + remaining[i+1:]) # Remove the number to try other possibilities current.pop() backtrack([], nums) return result Backtracking problems can be a bit mind-bending at first. I remember staring at my whiteboard for an embarrassingly long time trying to visualize the recursion tree. But once you get it, they become really satisfying to solve! Practice Problems: Practice Problems: Subsets (LeetCode #78) Permutations (LeetCode #46) N-Queens (LeetCode #51) Word Search (LeetCode #79) Subsets (LeetCode #78) LeetCode #78 Permutations (LeetCode #46) LeetCode #46 N-Queens (LeetCode #51) LeetCode #51 Word Search (LeetCode #79) LeetCode #79 Study Plan: How to Master These Patterns by 2025 Study Plan: How to Master These Patterns by 2025 Now that we've covered the most important patterns, let's talk strategy. Here's how I'd approach mastering these patterns if I were starting from scratch: Week 1-2: Focus on Sliding Window and Two Pointers Start with easy problems for each pattern Move to medium problems once comfortable Review solutions and optimize Week 3-4: Tree/Graph Traversal (DFS & BFS) Practice both recursive and iterative implementations Apply to tree problems first, then graph problems Make sure you understand the differences between DFS and BFS Week 5-6: Binary Search and Fast & Slow Pointers Master the basic template first Then tackle variations and edge cases Focus on problems that aren't obviously binary search at first glance Week 7-9: Dynamic Programming Start with simple 1D DP problems Move to 2D DP problems Practice recognizing when to use DP Week 10-12: Backtracking and Advanced Patterns Combine multiple patterns in complex problems Time yourself to simulate interview conditions Practice explaining your approach out loud Week 1-2: Focus on Sliding Window and Two Pointers Start with easy problems for each pattern Move to medium problems once comfortable Review solutions and optimize Week 1-2: Start with easy problems for each pattern Move to medium problems once comfortable Review solutions and optimize Start with easy problems for each pattern Move to medium problems once comfortable Review solutions and optimize Week 3-4: Tree/Graph Traversal (DFS & BFS) Practice both recursive and iterative implementations Apply to tree problems first, then graph problems Make sure you understand the differences between DFS and BFS Week 3-4: Practice both recursive and iterative implementations Apply to tree problems first, then graph problems Make sure you understand the differences between DFS and BFS Practice both recursive and iterative implementations Apply to tree problems first, then graph problems Make sure you understand the differences between DFS and BFS Week 5-6: Binary Search and Fast & Slow Pointers Master the basic template first Then tackle variations and edge cases Focus on problems that aren't obviously binary search at first glance Week 5-6: Master the basic template first Then tackle variations and edge cases Focus on problems that aren't obviously binary search at first glance Master the basic template first Then tackle variations and edge cases Focus on problems that aren't obviously binary search at first glance Week 7-9: Dynamic Programming Start with simple 1D DP problems Move to 2D DP problems Practice recognizing when to use DP Week 7-9: Start with simple 1D DP problems Move to 2D DP problems Practice recognizing when to use DP Start with simple 1D DP problems Move to 2D DP problems Practice recognizing when to use DP Week 10-12: Backtracking and Advanced Patterns Combine multiple patterns in complex problems Time yourself to simulate interview conditions Practice explaining your approach out loud Week 10-12: Combine multiple patterns in complex problems Time yourself to simulate interview conditions Practice explaining your approach out loud Combine multiple patterns in complex problems Time yourself to simulate interview conditions Practice explaining your approach out loud Remember, consistency beats intensity. I'd rather see you solve one problem thoroughly each day than binge 10 problems without understanding them. 🧠 It's Not Just About the Code It's Not Just About the Code Let me get real with you for a second. While these patterns are crucial, technical interviews are also about communication, problem-solving process, and handling pressure. I've bombed interviews where I actually knew the solution, just because I got nervous and couldn't articulate my thought process. And I've passed interviews where I didn't get the optimal solution but clearly explained my reasoning. So as you practice, don't just code-explain your approach out loud, consider trade-offs, analyze time and space complexity, and handle edge cases. These soft skills matter just as much as your algorithmic knowledge. algorithmic knowledge Well, that's all I've got for you today! I hope this guide helps you crush your coding interviews in 2025. Remember, you're not alone in this journey-we're all suffering through LeetCode together. Now go forth and conquer those patterns! suffering through LeetCode together Happy coding, and may the algorithmic gods be ever in your favor! Story by @saeedashifahmed from Rabbit Rank. Story by @saeedashifahmed from Rabbit Rank. @saeedashifahmed