Look for these signals in the problem:
- "Single number" among pairs: XOR cancels paired numbers, leaving the single one.
- "Power of " check: n & (n-1) equals only for powers of .
- "Count bits" or Hamming operations: Bit counting techniques like Brian Kernighan's algorithm.
- "Subset" enumeration with bitmask: Each bit represents inclusion/exclusion of an element.
- "No extra space" constraint: Bit manipulation often achieves space where other approaches need .