https://leetcode.com/problems/number-complement/
下面是这个问题的两个有效解决方案，为什么我在第一个解决方案中遇到整数溢出 mask 变量应该是int（而不是long），为什么我可以用int变量类型作为 mask 在第二种解决方案中

public int findComplement(int num) {

    // Make a copy of the number for length estimation purposes , because we will begin mutating it
    // directly
    int input = num;

    // Use a power of two mask to bring 1 at every position and flip in stages using XOR
    for (long mask = 1; mask <= input; mask *= 2) {
      num = (int) (num ^ mask);
    }

    return num;
  }

public int findComplement2(int num) {

    int save = num; 
    int mask = 1;

    while (save != 0) {
        num = num ^ mask;
        mask = mask << 1;
        save = save >> 1;
    }

   return num;

}