The Hamming distance between two integers is the number of positions at which the corresponding bits are different.
Input: x = 1, y = 4
Output: 2
Explanation:
1 (0 0 0 1)
4 (0 1 0 0)
↑ ↑
思路：就是求两数异或的二进制表示中有几个1.
如何求一个整数的二进制表示有几个一呢？
while(x){x=x&(x-1);count++;}
一个数减一之后，会把它最低位的一个1变成0，再与上它自己就会消掉最低位的1。