书上原题本想考的是大数问题，但是leetcode上要求的返回值是int，不可能出现当n过大时大数越界的问题， 失去了本题的意义。按大数问题处理，用string和递归来处理该问题。

public class Solution {public int[] PrintNumbers(int n) {List<int> resList = new List<int>();char[] bigNumber = new char[n];DFS(0, n, ref bigNumber, resList);if(resList.Contains(0))resList.Remove(0);return resList.ToArray();}private void DFS(int n, int target, ref char[]bigNumber, List<int> resList){if(n == target){string s = new string(bigNumber);resList.Add(int.Parse(s));return;}for(int i = 0 ; i < 10; i++){bigNumber[n] = char.Parse( i.ToString() );DFS(n + 1, target, ref bigNumber, resList);}}}