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
• QuickSelect

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