[LintCode] Count 1 in Binary

Count how many

1

in binary representation of a 32-bit integer.

Example

Given

32

, return

1

Given

5

, return

2

Given

1023

, return

9

Challenge

If the integer is n bits with m 1 bits. Can you do it in O(m) time?

常规解法：

class Solution {
public:
/**
* @param num: an integer
* @return: an integer, the number of ones in num
*/
int countOnes(int num) {
// write your code here
int cnt = 0;
while(num != 0){
cnt += (num&1);
num >>= 1;
}
return cnt;
}
};

使用STL的解法：

#include <bitset>
class Solution{
public:
/**
* @param num: an integer
* @return: an integer, the number of ones in num
*/
int countOnes(int num) {
// write your code here
bitset<sizeof(int)*8> bitvec(num);
return bitvec.count();
}
};