【LeetCode】Number Complement 解题报告

Question

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

Note

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

Example

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.

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.

Ways

找出一个二进制数的位反后的数。

本来想的是直接取反，也就是~num，但是试验下发现不行，因为有前面的0，取反后变成了1，然后就是个负数。

所以正确的方法应该是再与一下跟当前的数字的位数相同的全1的数字。其方法就是找出这个数字的第一个1，把后面的那些数字全部设为1.

没想到java竟然有这个库函数，那么就方便的多。

当然还有另外的方法，比如转成字符串，以此遍历到第一个1.

public class Solution {
    public int findComplement(int num) {
        return ~num & ((Integer.highestOneBit(num) << 1) - 1);
    }
}