【LeetCode】476. Number Complement

题目描述

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.

解决方案

C++

class Solution {
public:
    int findComplement(int num) {
        int b = 1;
        while(num > (b-1)){
            b <<= 1;
        }
        return (b - 1) - num ;
    }
};

C++

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

C++

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

java

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

解题思路
主要过程是求掩码，然后相减（也可以取反相与）。