leetcode-67- add binary

class Solution{
public:
	string (string a, string b) {
        string res = "";
        int len_a = a.length();
        int len_b = b.length();
        int carry = 0;
        while(len_a > 0 || len_b > 0){
            
        	int abit = len_a > 0 ? a[len_a-1] - '0' : 0;
        	int bbit = len_b > 0 ? b[len_b-1] - '0' : 0;
        	int cbit = carry > 0 ? 1 : 0;
            //字符串存在insert函数，可以直接调用
        	res.insert(res.begin(), '0' + ((abit + bbit + cbit) & 1));
        	carry = (abit + bbit + cbit) / 2;
        	len_a--;
        	len_b--;
        }
        if(carry > 0)
        	res.insert(res.begin(), '0' + carry);
        return res;
    }
};