338. Counting Bits

num. For every numbers i in the range 0 ≤ i ≤ num

Example:

For ​​num = 5​​ you should return ​​[0,1,1,2,1,2]​​.

由以前求i的二进制中1的个数方法得知，i的二进制比i&(i-1)多一个

class Solution {
public:
  vector<int> countBits(int num) {
    vector<int> res(num + 1);
    res[0] = 0;
    for (int i = 1; i <= num; i++){
      res[i] = res[i&(i - 1)] + 1;
    }
    return res;
  }
};