Sixteen, lexicographical sorting
16.1. Requirements for title design
Given an integer n, return all integers in the range [1, n] lexicographically.
You have to design an algorithm that is O(n) and uses O(1) extra space.
示例 1：
输入：n = 13
输出：[1,10,11,12,13,2,3,4,5,6,7,8,9]
示例 2：
输入：n = 2
输出：[1,2]
提示：
1 <= n <= 5 * 10^4
Source: LeetCode
Link: https://leetcode-cn.com/problems/lexicographical-numbers
16.2. Problem solving ideas
First, take the numbers from 1 to 9 for cur, and then compare them with n in turn. If n is greater than or equal to the number represented by cur, add cur to res, and then compare the numbers with cur as tens, hundreds, etc. with n Compare, if it is less than or equal to n, continue to add it to res, and finally output it.
16.3. Algorithms
class Solution {
List<Integer> res;
public List<Integer> lexicalOrder(int n) {
res = new ArrayList<>();
for (int i = 1; i <= 9; i++) {
dfs(i , n);
}
return res;
}
//cur当前位
public void dfs(int cur,int n){
//不符合条件
if (cur > n){
return;
}
//将符合条件的添加到res中
res.add(cur);
//将以cur为十位,百位等的数字比较并添加
for (int i = 0; i <= 9; i++) {
int nextNum = cur * 10 + i;
if (nextNum > n){
break;
}
//如果nextNum还小于n，再进行一次dfs
dfs(nextNum,n);
}
}
}
Reference video: B station up master Guo Guo wg