Leetcode_Add Binary

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) {                size_t  na = a.size();        int  nb = b.size();        int n = na > nb ? na : nb;                string result(n,'0');        int  ch= 0;        int i= n-1,ia=na-1,ib=nb-1;        for(;ia >=0 && ib >= 0; ){            int re = a[ia]-'0'+b[ib]-'0' + ch;            ch=re/2;            result[i]=re%2+'0';            ia--;            ib--;            i--;        }        //int re  = a[0]+b[0]-'0'+'0'+ch;        while(ia >=0){           int re = a[ia]-'0' +ch;           ch = re/2;           result[i] = re%2+'0';           ia--;           i--;        }                while(ib >=0){            int re = b[ib]-'0'+ch;            ch = re/2;            ib--;            result[i]= re%2+'0';            i--;        }              if(ch){            result.insert(0,"1");        }              return result;    }};