Maximum difference of zeros and ones in binary string (GFG)

int maxSubstring(string S)
{
    // Your code goes here
    int max_so_far = -1;
    int max_ending_here = 0;
    for(int i = 0; i < S.length(); i++) {
        int num = (S[i] == '1'? -1: 1);
        max_ending_here += num;
        if(max_ending_here > max_so_far) 
        max_so_far = max_ending_here;
        if(max_ending_here < 0) 
        max_ending_here = 0;
    }
    if(max_so_far)
    return max_so_far;
}