Leetcode算法比赛---- Lexicographical Numbers

问题描述

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.

Java算法实现

public class Solution {
    public List<Integer> lexicalOrder(int n) {
        List<Integer> list =new ArrayList<Integer>(n);
        if(n <= 9){
            for(int i = 1; i <= n; i++){
                list.add(i);
            }
        }
        else{
            for(int i = 1; i <= 9; i++){
                list.add(i);
                doAdd(list, i, n);//每次doAdd都是把以i开头的数字都加入List中
            }
        }
        return list;
    }

    public static void doAdd(List<Integer> list, int start, int n){
        int moreBit = start * 10;
        if(moreBit <= n){
            int ceil = n - moreBit;
            int ans;
            ceil = ceil > 9 ? 9 : ceil;
            for(int i = 0; i <= ceil; i++){
                ans = moreBit + i;
                list.add(ans);
                doAdd(list, ans, n);
            }
        }
    }
}