leetcode-67：二进制求和

输入:a = "11", b = "1"
输出："100"

输入：a = "1010", b = "1011"
输出："10101"

class Solution {
public:
    string addBinary(string a, string b) {
        int m=a.size(),n=b.size();
        int i=m-1,j=n-1;
        int add=0;
        int x,y=0;
        string res;
        while(i>=0||j>=0||add==1){
            x=i>=0?a[i]-'0':0;
            y=j>=0?b[j]-'0':0;
            int t=x+y+add;
            res+=t%2+'0';
            add=t/2;
            i--,j--;
        }
        reverse(res.begin(),res.end());
        return res;
    }
};