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.

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