【leetcode】Add Binary

class Solution {public:    string addBinary(string a, string b) {        // Note: The Solution object is instantiated only once and is reused by each test case.        reverse(a.begin(),a.end());        reverse(b.begin(),b.end());        string ans="";        int carry=0;        int len=max(a.size(),b.size());        for(int i=0;i<len;i++)        {            int ca=0;            if(i<a.size())                ca=a[i]-'0';            int cb=0;            if(i<b.size())                cb=b[i]-'0';                        int sum=ca+cb+carry;            ans+=sum%2+'0';//注意此处是+=，而不是ans[i]=            carry=sum/2;        }        if(carry==1)            ans+='1';        reverse(ans.begin(),ans.end());        return ans;    }};