LeetCode-Lexicographical Numbers

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:

public class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> resList = new ArrayList<Integer>();
        for (int i = 1; i < 10; i++) {
            lexicalOrderRecur(i, n, resList);
        }
        return resList;
    }

    public void lexicalOrderRecur(int cur, int n, List<Integer> resList) {
        if (cur > n) {
            return;
        }
        resList.add(cur);

        for (int i = 0; i < 10; i++) {
            if (10 * cur + i > n) {
                break;
            }
            lexicalOrderRecur(10 * cur + i, n, resList);
        }
    }
}