Leetcode 67 Add Binary

class Solution {
public:
    string addBinary(string a, string b) {
        reverse(a.begin(),a.end());
        reverse(b.begin(),b.end());
        if(a.size()>b.size()) swap(a,b);
        while(a.size()<b.size()) a+='0';
        int add=0;
        for(int i=0;i<a.size();i++)
        {
            int sum=add+a[i]+b[i]-'0'-'0';
            b[i]=sum%2+'0';
            add=sum/2;
        }
        if(add) b+='1';
        reverse(b.begin(),b.end());
        return b;
    }
};