If we are dealing with

**top/maximum/minimum/closest**'K' elements among 'N' elements, we will be using a**Heap**.If the given input is a

**sorted array**or a list, we will either be using**Binary Search**or the**Two Pointers**strategy.If we need to try all

**combinations**(or permutations) of the input, we can either use**Backtracking**or**Breadth First Search**.Most of the questions related to Trees or Graphs can be solved either through

**Breadth First Search**or**Depth First Search**.Every

**recursive**solution can be converted to an**iterative**solution using a**stack**.For a problem involving arrays, if there exists a solution in

**O(n^2)**time and**O(1)**space, there must exist two other solutionsUsing a

**HashMap**or**Set**for O(n) time and space.Using

**sorting**for O(n.log n) time O(1) space.

If a problem in asking for

**optimization**(e.g. maximization or minimization), we will be using**Dynamic Programming.**If we need to find some common

**substring**among a set of strings, we will be using a**HashMap**or**Trie**.If we need to

**search/manipulate**a bunch of strings then**Trie**will be the best data structure.If the problem is related to

**LinkedList**and we can't use extra space, then use the**Fast & Slow Pointer**approach.

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