386. Lexicographical Numbers

// LeetCode URL: https://leetcode.com/problems/lexicographical-numbers/
import java.util.ArrayList;
import java.util.List;

/**
 * Recursive DFS
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(log10 N)
 *
 * N = input number n.
 */
class Solution1 {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> result = new ArrayList<>();
        if (n <= 0) {
            return result;
        }

        for (int i = 1; i <= 9; i++) {
            if (i > n) {
                break;
            }
            dfsHelper(result, i, n);
        }

        return result;
    }

    private void dfsHelper(List<Integer> list, int cur, int n) {
        list.add(cur);

        cur *= 10;
        for (int i = 0; i <= 9; i++) {
            int newCur = cur + i;
            if (newCur > n) {
                return;
            }

            dfsHelper(list, newCur, n);
        }
    }
}

/**
 * Iterative
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1)
 *
 * N = input number n.
 */
class Solution2 {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> result = new ArrayList<>();
        if (n <= 0) {
            return result;
        }

        int cur = 1;

        for (int i = 1; i <= n; i++) {
            result.add(cur);

            if (cur * 10 <= n) {
                cur *= 10;
            } else {
                while (cur % 10 == 9 || cur == n) {
                    cur /= 10;
                }
                cur++;
            }
        }

        return result;
    }
}