LeetCode-Lexicographical Numbers

Posted 2020-08-05 LiBlog

Given an integer n, return 1 - n in lexicographical order.

For example, given 13, return: [1,10,11,12,13,2,3,4,5,6,7,8,9].

Please optimize your algorithm to use less time and space. The input size may be as large as 5,000,000.

Analysis:

The numbers are arranged in tree structure:

1

10 11 12 ... 19

100 101 ..... 109 | 110 111 .... 119 | .....

We just use DFS to go through the tree.

Solution:

 1 public class Solution {
 2     public List<Integer> lexicalOrder(int n) {
 3         List<Integer> resList = new ArrayList<Integer>();
 4         for (int i = 1; i < 10; i++) {
 5             lexicalOrderRecur(i, n, resList);
 6         }
 7         return resList;
 8     }
 9 
10     public void lexicalOrderRecur(int cur, int n, List<Integer> resList) {
11         if (cur > n) {
12             return;
13         }
14         resList.add(cur);
15 
16         for (int i = 0; i < 10; i++) {
17             if (10 * cur + i > n) {
18                 break;
19             }
20             lexicalOrderRecur(10 * cur + i, n, resList);
21         }
22     }
23 }