Leetcode386-Lexicographical Numbers

Description

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.

Solution

• Time Complexity: $O(N)$
• Space Complexity: $O(N)$

class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> res = new ArrayList<>();
        for (int i = 1; i <= 9; i++){
            helper(i, n, res);
        }
        return res;
    }
    
    private void helper(int cur, int n, List<Integer> res){
        if (cur > n){
            return;
        }
        
        res.add(cur);
        for (int i = 0; i <= 9; i++){
            int newInt = cur * 10 + i;
            if (newInt > n) return;
            helper(newInt, n, res);
        }
    }
}

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