LeetCode WiggleSort I && II

public class WiggleSort {

/*  Wiggle Sort I
Given an unsorted array nums, reorder it in-place such that
nums[0] <= nums[1] >= nums[2] <= nums[3]....

For example, given nums = [3, 5, 2, 1, 6, 4], one possible
answer is [1, 6, 2, 5, 3, 4].
*/

/*
*  Refer http://segmentfault.com/a/1190000003783283 *  Solution 1: Sort and two partition
*  Solution 2: Swap
*/
public void wiggleSortI(int[] nums) {
if(nums == null || nums.length <= 1) return;

int idx = 1;
while(idx < nums.length) {
if(idx % 2 == 0 && nums[idx] > nums[idx - 1]) { // nums[i] <= nums[i - 1]
swap(nums, idx, idx - 1);
} else if(idx % 2 == 1 && nums[idx] < nums[idx - 1]) { // nums[i] >= nums[i - 1]
swap(nums, idx, idx - 1);
}

idx++;
}
}

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

/*
leetcode Wiggle Sort II

Given an unsorted array nums, reorder it such that
nums[0] < nums[1] > nums[2] < nums[3]....

Example:
(1) Given nums = [1, 5, 1, 1, 6, 4], one possible answer is [1, 4, 1, 5, 1, 6].
(2) Given nums = [1, 3, 2, 2, 3, 1], one possible answer is [2, 3, 1, 3, 1, 2].

Note:
You may assume all input has valid answer.

Follow Up:
Can you do it in O(n) time and/or in-place with O(1) extra space?
*/

// refer: https://www.hrwhisper.me/leetcode-wiggle-sort-ii/ public void wiggleSort(int[] nums) {
if(nums == null || nums.length <= 1) return;

int mid = (nums.length - 1) / 2; // 0 based index

int midVal = findKthElement(nums, mid, 0, nums.length - 1);

int small = mid, large = nums.length - 1;

int[] temp = new int[nums.length];
for(int i = 0; i < nums.length; i++) {
if(i % 2 == 0) { // find next small
int smallVal = nums[small--];
temp[i] = smallVal;
} else {
int largeVal = nums[large--];
temp[i] = largeVal;
}
}

for(int i = 0; i < nums.length; i++) {
nums[i] = temp[i];
}
}

protected int findKthElement(int[] nums, int k, int start, int end) {
int partition = partition(nums, start, end);

if(partition == k)
return nums[k];
else if(partition < k) {
return findKthElement(nums, k, partition + 1, end);
} else {
return findKthElement(nums, k, start, partition - 1);
}
}

protected int partition(int[] nums, int start, int end) {
int pivot = start + (int)((end - start) * Math.random());
int pivotVal = nums[pivot];

swap(nums, pivot, end);

int pt = start;
for(int i = start; i < end; i++) {
if(nums[i] <= pivotVal) {
swap(nums, i, pt);
pt++;
}
}

swap(nums, end, pt);

return pt;
}

public static void main(String[] args) {
// TODO Auto-generated method stub
WiggleSort so = new WiggleSort();

int[] nums = {1,5,1,1,6,4 };

so.wiggleSort(nums);

for(int i : nums) {
System.out.println(i + " ");
}
}

}