67. Add Binary

 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;
     }
 }