public class Solution {

     public String addBinary(String a, String b) {

         char[] aa=a.toCharArray();

         char[] bb=b.toCharArray();

         int size=aa.length>=bb.length?aa.length:bb.length;

         int[] mm=new int[size];

         int c=0;

         int i=aa.length-1,j=bb.length-1,k=size-1;

         for(;i>=0&&j>=0;i--,j--,k--)

         {

             mm[k]=(aa[i]-'0'+bb[j]-'0'+c)%2;

             if(aa[i]-'0'+bb[j]-'0'+c>=2) c=1;

             else c=0;

         }

         while(i>=0)

         {

               mm[k]=(aa[i]-'0'+c)%2;

               if(aa[i]-'0'+c>=2) c=1;

               else c=0;

               k--;i--;

         }

         while(j>=0)

         {

             mm[k]=(bb[j]-'0'+c)%2;

             if(bb[j]-'0'+c>=2) c=1;

             else c=0;

              k--;j--;

         }

         String s="";

         if(c==1)

          s+=String.valueOf(c);

         for(int n=0;n<mm.length;n++)

         s=s+String.valueOf(mm[n]); 

         return s;

     }

 }