280. Wiggle Sort

Published on January 28, 2024

Question

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?

Algorithm

TBH, I was inspired by the explanation of first example
The description of the question illustrated that there must be a valid answer.
So I order the numbers and pick smallest and largest one by one to assemble a valid answer -> Code1
Actually I don't have to create a temp array to store the numbers. Swapping the nums[i+1] and nums[i] can be a valid answer -> code2
1. Or like what I did first and last, same idea;
A O(n) solution is that, we use greedy algorithm. A detailed expiation is here. ->code3

Code1

class Solution {
    public void wiggleSort(int[] nums) {
        Arrays.sort(nums);
        int[] res = new int[nums.length];
        int i = 0, j = nums.length - 1;
        int x = 0;
        while (i <= j) {
            if (i != j) {
                res[x++] = nums[i++];
                res[x++] = nums[j--];
            } else {
                res[x++] = nums[i++];
            }
        }
        for (i = 0; i < nums.length; i++) {
            nums[i] = res[i];
        }
    }
}

Code2

class Solution {
    public void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }

    public void wiggleSort(int[] nums) {
        Arrays.sort(nums);
        for (int i = 1; i < nums.length - 1; i += 2) {
            swap(nums, i, i + 1);
        }
    }
}

Code3

class Solution {
    public void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }

    public void wiggleSort(int[] nums) {
        for (int i = 0; i < nums.length - 1; i++) {
            if (((i % 2 == 0) && nums[i] > nums[i + 1])
                    || ((i % 2 == 1) && nums[i] < nums[i + 1])) {
                swap(nums, i, i + 1);
            }
        }
    }
}