Counting Bits.cpp

/*
Counting Bits
=============

Given an integer num, return an array of the number of 1's in the binary representation of every number in the range [0, num].

Example 1:
Input: num = 2
Output: [0,1,1]
Explanation:
0 --> 0
1 --> 1
2 --> 10

Example 2:
Input: num = 5
Output: [0,1,1,2,1,2]
Explanation:
0 --> 0
1 --> 1
2 --> 10
3 --> 11
4 --> 100
5 --> 101

Constraints:
0 <= num <= 105

Follow up:
It is very easy to come up with a solution with run time O(32n). Can you do it in linear time O(n) and possibly in a single pass?
Could you solve it in O(n) space complexity?
Can you do it without using any built-in function (i.e., like __builtin_popcount in C++)?
*/

class Solution
{
public:
  vector<int> countBits(int num)
  {
    vector<int> ans(num + 1, 0);
    for (int i = 1; i <= num; ++i)
    {
      ans[i] = ans[i / 2] + i % 2;
    }
    return ans;
  }
};