public static String add(String a, String b) {
    	int c=0;
    	StringBuilder rel=new StringBuilder();
    	for (int m=a.length(),n=b.length(), i=m-1, j=n-1;i>=0||j>=0||c!=0;--i,--j ) {
    		int x=i>=0?(a.charAt(i)-'0'):0;
    		int y=i>=0?(b.charAt(j)-'0'):0;
    		int sum=x+y+c;
    		rel.append(sum%10);
    		c=sum/10;
    	}
    	return rel.reverse().toString();
    }