LeetCode Beautiful Arrangement II

Posted 2020-10-09 Dylan_Java_NYC

原题链接在这里：https://leetcode.com/problems/beautiful-arrangement-ii/description/

题目：

Given two integers n and k, you need to construct a list which contains n different positive integers ranging from 1 to n and obeys the following requirement: 
Suppose this list is [a1, a2, a3, ... , an], then the list [|a1 - a2|, |a2 - a3|, |a3 - a4|, ... , |an-1 - an|] has exactly k distinct integers.

If there are multiple answers, print any of them.

Example 1:

Input: n = 3, k = 1
Output: [1, 2, 3]
Explanation: The [1, 2, 3] has three different positive integers ranging from 1 to 3, and the [1, 1] has exactly 1 distinct integer: 1.

Example 2:

Input: n = 3, k = 2
Output: [1, 3, 2]
Explanation: The [1, 3, 2] has three different positive integers ranging from 1 to 3, and the [2, 1] has exactly 2 distinct integers: 1 and 2.

Note:

1. The n and k are in the range 1 <= k < n <= 104.

题解：

头k-1个数按照1, n, 2, n-1, 3, n-2...的规律, diff的绝对值是n-1, n-2, n-3减小. 剩下的部分都按照diff是1增长.

Time Complexity: O(n).

Space: O(n), res space.

AC Java:

 1 class Solution {
 2     public int[] constructArray(int n, int k) {
 3         int [] res = new int[n];
 4         int l = 1;
 5         int r = n;
 6         int i = 0;
 7         while(l<=r){
 8             res[i] = (k <= 1) ? l++ : (k%2 == 1 ? l++ : r--);
 9             k--;
10             i++;
11         }
12         return res;
13     }
14 }

类似Beautiful Arrangement.