[LeetCode]Beautiful Arrangement II

题目描述：

LeetCode 667. Beautiful Arrangement II

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, ... , a_n], then the list [|a₁ - a₂|, |a₂ - a₃|, |a₃ - a₄|, ... , |a_n-1 - a_n|] has exactly k distinct integers.

If there are multiple answers, print any of them.

Example 1:

Input: n = 3, k = 1Output: [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 = 2Output: [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.

题目大意：

给定整数n和k，列表[a1, a2, a3, ... , an] = [1, 2, 3, ..., n]，重新排列，使得列表[|a1 - a2|, |a2 - a3|, |a3 - a4|, ... , |an-1 - an|] 恰好包含k个不同整数。

解题思路：

前k + 1个数采用摆动序列1, 1 + k, 2, k, ...

k + 2, ... n跟随其后

Python代码：

class Solution(object):
    def constructArray(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: List[int]
        """
        ans = [1]
        for x in range(k):
            m = ans[-1] + [1, -1][x % 2] * (k - x)
            ans.append(m)
        return ans + range(k + 2, n + 1)