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) {
        string res = "";
        int curry = 0;
        int i, j;
        for (i = a.length() - 1, j = b.length() - 1; i >= 0 && j >= 0; --i, --j)
        {
            if (((a[i] - '0') + (b[j] - '0')+curry) >=2)
            {
                res += ((curry + (a[i] - '0') + (b[j] - '0'))%2 + '0');
                curry = 1;
            }
            else
            {
                res += ((curry+'0' + ((a[i] - '0') + (b[j] - '0')) % 2) % 2 + '0');
                curry = 0;
            }
        }
        while (i >= 0)
        {
            if (((a[i] - '0')  + curry) >= 2)
            {
                res += ((curry + (a[i] - '0')) % 2 + '0');
                curry = 1;
            }
            else
            {
                res += ((curry + (a[i] - '0')) % 2 + '0');
                curry = 0;
            }
            --i;
        }
        while (j >= 0)
        {
            if (((b[j] - '0') + curry) >= 2)
            {
                res += ((curry + (b[j] - '0')) % 2 + '0');
                curry = 1;
            }
            else
            {
                res += ((curry + (b[j] - '0')) % 2 + '0');
                curry = 0;
            }
            --j;
        }
        if (curry == 1)
            res += '1';
        reverse(res.begin(), res.end());
        return res;
    }
};