Rearrange a list of integers so that elements alternate between less-than and greater-than their neighbors, creating a zigzag pattern: a < b > c < d > e...
For example, given [4, 3, 7, 8, 6, 2, 1], a valid zigzag arrangement would be [3, 7, 4, 8, 2, 6, 1] where 3 < 7 > 4 < 8 > 2 < 6 > 1.
The method should modify the list in-place and return it. For null input, return null. For lists with one or zero elements, return the list unchanged.
Examples:
Input: nums = [1, 4, 3, 2]
Output: [1, 4, 2, 3] (or any valid zigzag pattern)
Explanation: The output follows the pattern 1 < 4 > 2 < 3
Input: nums = [4, 3, 7, 8, 6, 2, 1]
Output: [3, 7, 4, 8, 2, 6, 1] (or any valid zigzag pattern)
Explanation: Each element alternates between less-than and greater-than its neighbors
Input: nums = [1]
Output: [1]
Explanation: Single element is already in valid zigzag pattern
Input: nums = null
Output: null
Explanation: Null input returns null
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