If input array is sorted then - Binary search - Two pointers
If asked for all permutations/subsets then - Backtracking
If given a tree then - DFS - BFS
If given a graph then - DFS - BFS
If given a linked list then - Two pointers
If recursion is banned then - Stack
If must solve in-place then - Swap corresponding values - Store one or more different values in the same pointer
If asked for maximum/minimum subarray/subset/options then - Dynamic programming
If asked for top/least K items then - Heap
If asked for common strings then - Map - Trie
Else - Map/Set for O(1) time & O(n) space - Sort input for O(nlogn) time and O(1) space