LeetCode之Add Binary

class Solution {
public:
    string addBinary(string a, string b) {
        int i(a.size() - 1), j(b.size() - 1);
        int carry(0);
        string res("");
        while(i >= 0 && j >= 0){
            int num = (a[i] - '0') + (b[j] - '0') + carry;
            char c = '0' + num%2;
            carry = num/2;
            res += c;
            --i;
            --j;
        }
        while(i >= 0){
            int num = (a[i] - '0') + carry;
            char c = '0' + num%2;
            carry = num/2;
            res += c;
            --i;
        }
        while(j >= 0){
            int num = (b[j] - '0') + carry;
            char c = '0' + num%2;
            carry = num/2;
            res += c;
            --j;
        }
        if(carry == 1) res += '1';
        reverse(res.begin(), res.end());
        return res;
    }
};