Add Binary

Given two binary strings, return their sum (also a binary string).
For example,
a = “11”
b = “1”
Return ”100”.
题意：将两个二进制相加
分析：
字符串要先转化为整数，然后再连接字符串。

class Solution {
public:
    string addBinary(string a, string b) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int flag = 0;

        int aIndex = a.size() - 1;
        int bIndex = b.size() - 1;

        string c;

        while(aIndex >= 0 && bIndex >= 0)
        {
            int num = a[aIndex] - '0' + (b[bIndex] - '0') + flag;
            flag = num / 2;
            num %= 2;

            c = (char)(num + '0') + c;

            aIndex--;
            bIndex--;
        }

        while(aIndex >= 0)
        {
            int num = a[aIndex] - '0' + flag;
            flag = num / 2;
            num %= 2;

            c = (char)(num + '0') + c;

            aIndex--;
        }

        while(bIndex >= 0)
        {
            int num = b[bIndex] - '0' + flag;
            flag = num / 2;
            num %= 2;

            c = (char)(num + '0') + c;

            bIndex--;
        }

        if (flag > 0)
            c = (char)(flag + '0') + c;

        return c;
    }
};