LeetCode 0338 - Counting Bits

Counting Bits

Desicription

Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1’s in their binary representation and return them as an array.

Example 1:

Input: 2
Output: [0,1,1]

Example 2:

Input: 5
Output: [0,1,1,2,1,2]

Follow up:

• It is very easy to come up with a solution with run time O(n*sizeof(integer)). But can you do it in linear time O(n) /possibly in a single pass?
• Space complexity should be O(n).
• Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.

Solution

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