by Sean Prashad
A collection of 178 questions grouped by pattern to help you prepare for coding interviews.
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About
In 2019, as a broke college student who couldn't afford premium interview resources, I spent countless hours searching for free materials and teaching myself React to build Leetcode Patterns.
I believe everyone deserves access to high-quality interview material - regardless of their financial situation. It's why I chose to make this website free and open source.
Best of luck on your journey!
Helpful Tips
Based on the problem constraints, use these heuristics to identify possible approaches when unsure.
| Condition | Approach |
|---|---|
| Arrays & Strings | |
| If input array is sorted | Binary search, Two pointers |
If need O(1) lookup | Hash table, Hash set |
| If must solve in-place | Swap corresponding values, Store multiple values in the same pointer |
| If asked for common strings | Map, Trie |
| If asked to count bits or use XOR | Bit manipulation |
| Subarrays & Sequences | |
| If asked for max/min subarray/subset | Dynamic programming, Sliding window |
| If asked for sliding window max/min | Monotonic queue |
| If asked for next greater/smaller element | Monotonic stack |
| If need range sum/frequency queries | Prefix sum, Binary indexed tree, Segment tree |
| Trees & Graphs | |
| If given a tree | DFS, BFS |
| If given a graph | DFS, BFS, Union-Find |
| If given a matrix | BFS, DFS, Dynamic programming |
| If asked for connectivity/grouping | Union-Find, DFS |
| If asked for ordering/scheduling | Topological sort |
| Linked Lists & Stacks | |
| If given a linked list | Two pointers |
| If recursion is banned | Stack |
| Sorting & Intervals | |
If asked for top/least K items | Heap, Quickselect, Bucket sort |
| If asked to merge sorted lists/intervals | Merge sort, Heap |
| If asked for overlapping intervals | Sorting, Sweep line |
| If given a stream of data | Heap, Design |
| Optimization | |
| If asked for all permutations/subsets | Backtracking |
| If need to count/divide optimally | Greedy, Dynamic programming |
| General | |
| Else | Map/Set for O(1) time & O(n) space, Sort input for O(nlogn) time and O(1) space |
Acknowledgements
Leetcode Patterns wouldn't exist without the following resources:
