Question

Given a positive integer, output its complement number. The complement strategy is to flip the bits of its binary representation.

Note:

1. The given integer is guaranteed to fit within the range of a 32-bit signed integer.
2. You could assume no leading zero bit in the integer’s binary representation.

Example 1:

Input: 5 Output: 2 Explanation: The binary representation of 5 is 101 (no leading zero bits), and its complement is 010. So you need to output 2.

Example 2:

Input: 1 Output: 0 Explanation: The binary representation of 1 is 1 (no leading zero bits), and its complement is 0. So you need to output 0.

Difficulty:Easy

Category:Bit-Manipulation

Analyze

This idea is come from the leetcode discuss. The actual process for the solution like:

num           = 00000101
mask          = 11111000
mask ^ num    = 11111101
~(mask ^ num) = 00000010

Solution

class Solution {
 public:
  int findComplement(int num) {
    int mask = ~0;
    int bit = 0;
    while (mask & num) mask &= ~(1 << bit++);
    return ~(mask ^ num);
  }
};