67. Add Binary(unsolved)

Given two binary strings, return their sum (also a binary string).

For example,
a = “11”
b = “1”
Return “100”.

class Solution {
public:
    string addBinary(string a, string b) {
        int alen=a.size()-1;
        int blen=b.size()-1;
        string result="";
        int carry=0;
        while(alen>=0||blen>=0)
        {
            int p=alen>=0?a[alen--]-'0':0;
            int q=blen>=0?b[blen--]-'0':0;
            int sum=p+q+carry;
            result=to_string(sum%2)+result;
            carry=sum/2;
        }
        result=carry==1?"1"+result:result;
        return result;  
    }
};

二刷时，要特别注意，string的起始是左边，也即是1010的s[0]是1