Java Program to Sort an array in wave form

Last Updated : 23 Jul, 2025

Given an unsorted array of integers, sort the array into a wave-like array. An array 'arr[0..n-1]' is sorted in wave form if arr[0] >= arr[1] <= arr[2] >= arr[3] <= arr[4] >= .....

Examples:

 Input: arr[] = {10, 5, 6, 3, 2, 20, 100, 80}
 Output: arr[] = {10, 5, 6, 2, 20, 3, 100, 80} OR
 {20, 5, 10, 2, 80, 6, 100, 3} OR
 any other array that is in wave form

 Input: arr[] = {20, 10, 8, 6, 4, 2}
 Output: arr[] = {20, 8, 10, 4, 6, 2} OR
 {10, 8, 20, 2, 6, 4} OR
 any other array that is in wave form

 Input: arr[] = {2, 4, 6, 8, 10, 20}
 Output: arr[] = {4, 2, 8, 6, 20, 10} OR
 any other array that is in wave form

 Input: arr[] = {3, 6, 5, 10, 7, 20}
 Output: arr[] = {6, 3, 10, 5, 20, 7} OR
 any other array that is in wave form

A Simple Solution is to use sorting. First sort the input array, then swap all adjacent elements.
For example, let the input array be {3, 6, 5, 10, 7, 20}. After sorting, we get {3, 5, 6, 7, 10, 20}. After swapping adjacent elements, we get {5, 3, 7, 6, 20, 10}.

Below are implementations of this simple approach.

Output

2 1 10 5 49 23 90

Time complexity: O(n Log n), if a O(nLogn) sorting algorithm like Merge Sort, Heap Sort, .. etc is used.
Auxiliary Space: O(1)

Another approach:
This can be done in O(n) time by doing a single traversal of given array. The idea is based on the fact that if we make sure that all even-positioned (at index 0, 2, 4, ..) elements are greater than their adjacent odd elements, we don't need to worry about odd positioned elements.

The following are simple steps.

Traverse all even positioned elements of input array, and do following.
- If current element is smaller than previous odd element, swap previous and current.
- If current element is smaller than next odd element, swap next and current.

Below are implementations of the above simple algorithm.

Output