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Clean C++ solution, beats most submissions. Use a hash map from value to index (or count). For each element check whether its complement is already in the map, then insert it. That makes it O(n) instead of the O(n²) brute force.

Intuition + complexity breakdown for Happy Number. Use a hash map from value to index (or count). For each element check whether its complement is already in the map, then insert it. That makes it O(n) instead of the O(n²) brute force.

Simple approach explained for beginners. Use a hash map from value to index (or count). For each element check whether its complement is already in the map, then insert it. That makes it O(n) instead of the O(n²) brute force.