Correct or Not

One night Mr.X and Mr.Y were playing a game. To test Mr.Y's intelligence Mr.X gives him an interesting problem to solve. He gives him a number N and asks him to swap the digits in the number such that a smallest number M is obtained without leading zeroes. Mr.Y thinks this is an easy problem to solve and immediately gives the answer. The number can be large therefore Mr.X wants to check Mr.Y's