Tries & Bit Manipulation

Every value appears twice except one; find it in O(1) space.

Count the set bits in a non-negative integer.

Set-bit counts for every number from 0 to n, in linear time.

Find the one number from 0..n missing from the list.

Build a trie class with insert, search and starts_with.

A word dictionary whose search supports '.' as a wildcard letter.

Largest a ^ b over all pairs, faster than checking every pair.