Lintcode: Add Binary

C++

class Solution {
public:
    /**
     * @param a a number
     * @param b a number
     * @return the result
     */
    string addBinary(string& a, string& b) {
        // Write your code here
        if (a == "0") {
            return b;
        }
        if (b == "0") {
            return a;
        }
        string result;
        int carry = 0;
        int ai, bj, sum;
        char val;
        for (int i = a.size()-1, j = b.size()-1; i >= 0 || j >= 0; i--, j--) {
            ai = i >= 0?a[i]-'0':0;
            bj = j >= 0?b[j]-'0':0;
            sum = ai+bj+carry;
            val = sum%2?'1':'0';
            carry = sum/2;
            result.insert(result.begin(), val);
        }
        if (carry == 1) {
            result.insert(result.begin(), '1');
        }
        return result;
    }
};