LeetCode-in-Java
386. Lexicographical Numbers
Medium
Given an integer n, return all the numbers in the range [1, n] sorted in lexicographical order.
You must write an algorithm that runs in O(n) time and uses O(1) extra space.
Example 1:
Input: n = 13
Output: [1,10,11,12,13,2,3,4,5,6,7,8,9]
Example 2:
Input: n = 2
Output: [1,2]
Constraints:
1 <= n <= 5 * 104
Solution
import java.util.ArrayList;
import java.util.List;
public class Solution {
public List<Integer> lexicalOrder(int n) {
List<Integer> lst = new ArrayList<>();
dfs(lst, n, 0);
return lst;
}
private void dfs(List<Integer> lst, int n, int num) {
for (int i = 0; i <= 9; i++) {
int cur = 10 * num + i;
// get rid of 0
if (cur == 0) {
continue;
}
// when larger than n, return to the previous level
if (cur > n) {
return;
}
lst.add(cur);
dfs(lst, n, cur);
}
}
}