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# 0280. Wiggle Sort

<https://leetcode.com/problems/wiggle-sort>

## Description

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

You may assume the input array always has a valid answer.

**Example 1:**

```
**Input:** nums = [3,5,2,1,6,4]
**Output:** [3,5,1,6,2,4]
**Explanation:** [1,6,2,5,3,4] is also accepted.
```

**Example 2:**

```
**Input:** nums = [6,6,5,6,3,8]
**Output:** [6,6,5,6,3,8]
```

**Constraints:**

* `1 <= nums.length <= 5 * 104`
* `0 <= nums[i] <= 104`
* It is guaranteed that there will be an answer for the given input `nums`.

**Follow up:** Could you do it without sorting the array?

## ac1: in-place odd/even check

This is kinda "aha" brain teaser. See proof: <https://leetcode.com/problems/wiggle-sort/discuss/71693/My-explanations-of-the-best-voted-Algo/112986>.

```java
class Solution {
    public void wiggleSort(int[] nums) {
        // edge cases
        if (nums.length < 2) return;

        // body
        for (int i = 0; i < nums.length - 1; i++) {
            if (i % 2 == 0 && nums[i] > nums[i+1]) swap(nums, i, i+1);
            if (i % 2 == 1 && nums[i] < nums[i+1]) swap(nums, i, i+1);
        }
    }
    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

// O(N) time, O(1) space
```

## ac2: quickselect partition

Same as [324-Wiggle Sort II](https://github.com/jaywinhuang/leetcode/blob/master/mysolutions/324-wiggle-sort-ii.md)