Leetcode 191. Number of 1 Bits

Write a function that takes an unsigned integer and returns the number of ’1' bits it has.
For example, the 32-bit integer ’11' has binary representation
00000000000000000000000000001011, so the function should return 3.

思路：
不断右移数字，与1，查找为1的个数；或者mask不断左移，左移不需要考虑算术右移或者逻辑右移的问题。

public static int hammingWeight1(int n) {
    int res = 0;
    while (n != 0) {
        res += n & 1;
        n >>>= 1;
    }
    return res;
}


public int hammingWeight(int n) {
int bits = 0;
int mask = 1;
for (int i = 0; i < 32; i++) {
    if ((n & mask) != 0) {
        bits++;
    }
    mask <<= 1;
}
return bits;
}

非按位查找，只找数字中是1的位，可以利用n&(n-1)会消去n最右边1的特性。

public static int hammingWeight2(int n) {
    int res = 0;
    while (n != 0) {
        res += 1;
        n &= (n-1);
    }
    return res;
}