[leetcode]476. Number Complement

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.

分析：

求补数，即将原来数字的二进制形式的0取1,1取0之后的数，需要注意的是没有前置0。由于高位0不能变为1，因此从高位开始遍历，遇到第一个1开始，遇到1之后开始每位取反，即与1异或即可。

class Solution {
public:
    int findComplement(int num) {
        bool start = false;
        for (int i = 31; i >= 0; --i) {
            if (num & (1 << i)) 
                start = true;
            if (start) 
                num ^= (1 << i);
        }
        return num;
    }
};