Lexicographical Numbers 386

Question

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

典型的backtrack问题。不断 * 10直到大于n，再返回上一层继续往下一个数。

代码如下：

public class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> res = new ArrayList<Integer>();

        for (int i = 1; i < 10; i++) {
            dfs(n, i, res);
        }
        return res;
    }

    private void dfs(int n, int curt, List<Integer> res) {
        if (curt > n) {
            return;
        }
        res.add(curt);
        for (int i = 0; i < 10; i++) {
            if (curt * 10 + i > n) {
                return;
            } else {
                dfs(n, curt * 10 + i, res);
            }
        }
    }
}