LeetCode--Counting Bits

Idea 1:

    1. Determine whether each bit is 1 by shifting

    2. Iterate over all numbers

class Solution {
    public int[] countBits(int num) {
        int[] res=new int[num+1];
        
        for(int i=0;i<=num;i++){
            res[i]=binaryToDecimal(i);
        }
        
        return res;
    }
    
    public int binaryToDecimal(int n){
        int res=0;
        
        for(int i = 31;i >= 0; i--){
            if((n>>>i & 1)==1){
                res++;
            }
        }
        
        return res;
    }
}

Idea 2:

    I haven't understood it yet, but the space complexity is lower than the above one

public int[] countBits(int num) {
    int [] f = new int [num + 1];
    for (int i=1; i<=num; i++) f[i] = f[i >> 1] + (i & 1);
    return f;
}