【LeetCode】Add Binary

class Solution {
public:
    string addBinary(string a, string b) {
        int aindex = a.size() - 1;
	int bindex = b.size() - 1;
	string c;
	int flag = 0,num;
	while (aindex>=0 && bindex>=0)
	{
		num = a[aindex] - '0' + b[bindex] - '0'+flag;
		flag = num / 2;
		num %= 2;
		c = (char(num + '0')) + c;

		aindex--;
		bindex--;

	}
	while (aindex>=0)
	{
		num = a[aindex] - '0' + flag;
		flag = num / 2;
		num %= 2;
		c = (char(num + '0')) + c;
		aindex--;
	}
	while (bindex >= 0)
	{
		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;
    }
};