Print Numbers by Recursion

    1. public class Solution {
    2.     /**
    3.      * @param n: An integer.
    4.      * return : An array storing 1 to the largest number with n digits.
    5.      */
    6.         List<Integer> result = new ArrayList<Integer>();
    7.         if (n <= 0) {
    8.             return result;
    9.         }
    10.         helper(n, result);
    12.     }
    14.     private void helper(int n, List<Integer> ret) {
    15.         if (n == 0) return;
    16.         helper(n - 1, ret);
    17.         // current base such as 10, 20, 30...
    18.         // get List size before for loop
    19.         int size = ret.size();
    20.         for (int i = 1; i < 10; i++) {
    21.             // add 10, 100, 1000...
    23.             for (int j = 0; j < size; j++) {
    24.                 // add 11, 12, 13...
    25.                 ret.add(ret.get(j) + base * i);
    26.             }
    27.         }
    28.     }
    29. }