Given two binary strings, return their sum (also a binary string). For example, a = "11" b = "1" Return "100". http://leetcode.com/onlinejudge#question_67

leetcode_add_binary.cpp

class Solution {
public:
    string addBinary(string a, string b) {
        string ret;
        int lena = a.length();
        int lenb = b.length();
        int t = 0;
        int i = lena - 1;
        int j = lenb - 1;
        while (i >= 0 || j >= 0) {
            int s = t;
            if (i >= 0)
                s += a[i--] - '0';
            if (j >= 0)
                s += b[j--] - '0';
            if (s == 0 || s == 2)
                ret.append("0");
            else
                ret.append("1");
            if (s >= 2)
                t = 1;
            else
                t = 0;
        }
        if (t == 1)
            ret.append("1");
        reverse(ret.begin(), ret.end());
        return ret;
    }
};