Is Wiggle Sort easy or hard?

Wiggle Sort is considered a medium-level problem. The brute-force idea using sorting is straightforward, but recognizing that the wiggle condition can be enforced with a single greedy pass requires stronger problem-solving insight.

Wiggle Sort Python/Java solution

In Python or Java, implement a single loop from index 1 to n-1 and compare each element with its predecessor. Depending on whether the index is odd or even, swap the pair when the inequality condition is violated. The logic is identical across languages and runs in O(n) time.

How to solve Wiggle Sort in O(n)?

Traverse the array from index 1 to n-1. For each index i, check the wiggle condition: if i is odd and nums[i] nums[i-1], swap them. Each swap fixes the local ordering without breaking previous pairs, so the entire array becomes wiggle-sorted in one pass.

What is the best approach for Wiggle Sort?

The best approach is a one-pass greedy traversal. Iterate through the array and enforce the wiggle rule at each index: odd indices should be greater than or equal to the previous element, and even indices should be less than or equal to the previous element. Swap adjacent elements whenever the rule is violated. This runs in O(n) time with O(1) extra space.

Is Wiggle Sort asked at Google/Amazon/Meta?

Wiggle-style array rearrangement problems appear in interviews at companies like Google, Amazon, and Meta. They test understanding of greedy strategies, in-place array manipulation, and reasoning about local constraints that maintain global ordering patterns.

What data structure is used in Wiggle Sort?

The problem primarily uses arrays with in-place element swaps. The optimal solution relies on a greedy traversal rather than additional data structures, which keeps the space complexity at O(1). Sorting-based solutions may use the language’s built-in sorting implementation.

What is the time complexity of Wiggle Sort?

Two common solutions exist. Sorting the array and then swapping adjacent elements takes O(n log n) time due to the sort step. The optimal greedy solution scans the array once and performs constant-time swaps, resulting in O(n) time and O(1) space.

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280. Wiggle Sort

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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 solve the problem in O(n) time complexity?

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