[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) {
        string maxS,minS;
   
   if(a.size()>b.size())
   {
     maxS = a;
     minS = b;
   
   }
   else
   {
     maxS = b;
     minS = a;
   
   }

   int len1,len2;
   len1 = maxS.size();
   len2 = minS.size();
   int flag = 0;
   string res(len1+1,'0');

   int i = len1-1;
   int j = len2-1;
   int z = len1;
   while(j>=0)
   {
      int num = maxS[i--]-'0'+minS[j--]-'0'+flag;
      if(num>=2)
      {
        num -= 2;
        flag = 1;
      }
      else
      {
        flag = 0;
      }
      res[z--] = num + '0';

   }
   while(i>=0)
   {
     int num = maxS[i--]-'0'+flag;
     if(num>=2)
      {
        num -= 2;
        flag = 1;
      }
      else
      {
        flag = 0;
      }
      res[z--] = num + '0';
   
   }
   if(flag == 1)
   {
       res[z] = '1';
       return res;
   }
   else
   {
       return res.substr(1);
   
   }
    }
};