Leetcode @Wiggle Sort I, II

Given an unsorted array nums, reorder it such that nums[0] < nums[1] > nums[2] < nums[3]....

Example 1:

Input: nums = [1, 5, 1, 1, 6, 4]
Output: One possible answer is [1, 4, 1, 5, 1, 6].

Example 2:

Input: nums = [1, 3, 2, 2, 3, 1]
Output: One possible answer is [2, 3, 1, 3, 1, 2].

Note:
You may assume all input has valid answer.

Follow Up:
Can you do it in O(n) time and/or in-place with O(1) extra space?

波浪型排序，Wiggle Sort I 很简单，因为可以相等，所以把整个数组先排序，再2 pointers两个互相swap就可以了。

Wiggle Sort II 比较难。破题点在哪里？这是个什么类型的问题？