[LeetCode] Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

class Solution {
public:
    string addBinary(string a, string b) {
        int lenA = a.length();
        int lenB = b.length();
        
        int i = 0;
        int overflow = 0;
        string res;
        while(i<lenA || i<lenB)
        {
            int a1 = i<lenA ? a[lenA - 1 - i] - '0' : 0;
            int a2 = i<lenB ? b[lenB - 1 - i] - '0' : 0;
            int sum = a1 + a2 + overflow;
            overflow = sum > 1 ? 1: 0;
            char c = sum %2 +'0';
            res = c + res;
            i++;
        }
        if(overflow) res = '1' + res;
        return res;
    }
};