Reverse both of strings
char [ ] ch = str1 . ToCharArray ( ) ; Array . Reverse ( ch ) ; str1 = new string ( ch ) ; char [ ] ch1 = str2 . ToCharArray ( ) ; Array . Reverse ( ch1 ) ; str2 = new string ( ch1 ) ; int carry = 0 ; for ( int i = 0 ; i < n1 ; i ++ ) {