LeetCode cheatsheet

P𝗋𝗈𝖻𝗅𝖾𝗆 S𝗈𝗅𝗏𝗂𝗇𝗀 Tips

1. If an input array is sorted then

a. Binary search O(log n)

b. Two pointers O(n)

2. If asked for all permutations/subsets then

a. Backtracking (First preference)

b. Breadth First Search.

3. If given a tree or graphs then

a. DFS  (Preorder, Inorder, Postorder): O(n)

b. BFS (Level Order): O(n)

4. If Binary search tree:

a. - Left < Cur < Right: O(log n)

b. - Inorder Traversal visits the nodes in ascending (sorted) order: O(n

5. If given a linked list then

a. Two pointers

6. If recursion is banned then

a.   Convert to iterative solution using Stack

7. If must solve in-place then

a. Swap corresponding values

b. Store one or more different values in the same pointer

8. If asked for maximum/minimum subarray/subset/options then

a. Dynamic programming

9. If asked for top/maximum/minimum/closest/least K items then

a. Heap

10. If asked for common strings among a set of strings

a. Hash Map

b. Trie

11. If we need to search/manipulate a bunch of strings:

a. Trie

12. For a problem involving arrays, if there exists a solution in O(n^2) time and O(1) space, there                  must exist two other solutions:

a. Use a HashMap or a Set for O(n) time and O(n) space

b. Sort input for O(n log n) time and O(1) space

13. Else

a. Use a HashMap or a Set for O(n) time and O(n) space

                Sort input for O(n log n) time and O(1) space

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