67. Add Binary

还是模拟，模拟就完事儿了

class Solution {
public:
    string addBinary(string a, string b) {
        int s_it_a=a.length()-1;
        int s_it_b=b.length()-1;
        string res="";
        int carry=0;
        while(s_it_a>=0||s_it_b>=0){
            int c_a=s_it_a>=0?a[s_it_a]-'0':0;
            int c_b=s_it_b>=0?b[s_it_b]-'0':0;
            int c_k=c_a+c_b+carry;
            carry=0;
            if(c_k>1){
                carry=1;
                c_k=c_k-2;
            }
            res=(char)('0'+c_k)+res;
            --s_it_a;
            --s_it_b;
        }
        while(carry){
            res='1'+res;
            --carry;
        }
        return res;
    }
};