14 Coding Patterns That Solve 80% of Interview Problems

14 Coding Patterns That Solve 80% of Interview Problems

The best way to prepare for coding interviews isn't to solve 500 LeetCode problems. It's to recognise the 14 patterns that most problems reduce to.

The 14 Patterns

#	Pattern	Example Problems
1	Two Pointers	Two Sum (sorted), Container With Most Water
2	Sliding Window	Longest Substring Without Repeating Characters
3	Fast & Slow Pointers	Linked List Cycle, Find Middle of List
4	Merge Intervals	Merge Intervals, Insert Interval
5	Cyclic Sort	Find All Missing Numbers, Find Duplicate Number
6	In-Place Linked List Reversal	Reverse Linked List, Reverse Sub-list
7	Tree BFS	Binary Tree Level Order Traversal
8	Tree DFS	Path Sum, All Root-to-Leaf Paths
9	Two Heaps	Find Median from Data Stream
10	Subsets (Backtracking)	Subsets, Permutations, Combinations
11	Modified Binary Search	Search in Rotated Array, Find Min in Rotated Array
12	Top-K Elements	K Largest Elements, K Most Frequent
13	K-Way Merge	Merge K Sorted Lists
14	Dynamic Programming	Coin Change, Longest Common Subsequence

How to identify patterns

Sliding Window → "subarray/substring with constraint" + contiguous elements

Two Pointers → sorted array + target sum, or palindrome/reverse check

Tree DFS → process individual node, return something to parent (path problems)

Tree BFS → level-by-level, find something at a specific depth

Top-K → K largest/smallest, and sort is too slow → min-heap of size K

DP → count ways / optimal value + overlapping subproblems

Example: how to read a problem

"Find the length of the longest substring without repeating characters"

• "substring" → contiguous → sliding window
• "without repeating" → constraint on window contents
• Template: expand right pointer, shrink left pointer when constraint violated

Once you see the pattern, the code writes itself.