67. Add Binary

注意进位：

class Solution {
public:
    string addBinary(string s, string t) {
                    int flag = 0;
            string res;
            int i = s.size() - 1, j = t.size() - 1;

            while (i >= 0 && j >= 0)
            {
                int sum = (s[i] - '0') +(t[j] - '0') + flag;
                flag = sum / 2;  //进位
		        sum %= 2;//和 0/1

                res = char(sum + '0') + res;
                i--;
                j--;
            }
            while (i >= 0)
            {
                int sum = (s[i] - '0') + flag;
                flag = sum / 2;
	        	sum %= 2;
                res = char(sum + '0') + res;
                i--;
            }
            while (j >= 0)
            {
                int sum = (t[j] - '0') + flag;
                flag = sum / 2;
		        sum %= 2;
                res = char(sum + '0') + res;
                j--;
            }
            if (flag != 0)
                res = char(flag + '0') + res;
            return res;
        
    }
};