DSA-25-Questions-Patterns.md

✅ Arrays / Two Pointers / Hashing

3Sum — Medium

Use sorting + two pointers to find triplets that sum to zero.

Container With Most Water — Medium

Two pointers from both ends, greedily track max area.

Longest Consecutive Sequence — Hard

Use a HashSet to check sequence starts efficiently.

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🔤 Strings / Sliding Window / Hashing

Longest Substring Without Repeating Characters — Medium

Sliding window + HashSet/Map.

Minimum Window Substring — Hard

Sliding window + two pointers + character frequency count.

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📚 Stack

Valid Parentheses — Easy

Stack to match open/close brackets.

Daily Temperatures — Medium

Monotonic stack pattern.

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🔗 Linked List

Reverse Linked List — Easy

Iterative or recursive reversal.

Linked List Cycle — Easy

Floyd’s Tortoise and Hare.

Merge Two Sorted Lists — Easy

Merge like merge sort.

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🌲 Trees / Binary Trees / BST

Binary Tree Level Order Traversal — Medium

BFS using queue.

Diameter of Binary Tree — Easy

Post-order DFS to compute depths.

Lowest Common Ancestor of a BST — Easy

Leverage BST properties in recursion.

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🧩 Backtracking

Subsets — Medium

Classic power set generation.

Combination Sum — Medium

Backtracking with choices and pruning.

Word Search — Medium

DFS with backtracking on grid.

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🌐 Graphs / BFS / DFS / Union-Find

Number of Islands — Medium

DFS or BFS flood-fill.

Clone Graph — Medium

DFS with hashmap for visited nodes.

Course Schedule — Medium

Topological sort via DFS or Kahn’s algo.

Redundant Connection — Medium

Union-Find to detect cycle.

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🔁 Dynamic Programming

Climbing Stairs — Easy

Classic DP or Fibonacci optimization.

House Robber — Medium

DP over choices: rob or skip.

Longest Palindromic Substring — Medium

Expand around center or DP table.

Coin Change — Medium

Unbounded knapsack DP.

Partition Equal Subset Sum — Medium

Subset sum DP (1D array can optimize).