215. Kth Largest Element in an Array

#### QUESTION:

Given an integer array nums and an integer k, return the kth largest element in the array.

Note that it is the kth largest element in the sorted order, not the kth distinct element.

You must solve it in O(n) time complexity.

Example 1:

``````Input: nums = [3,2,1,5,6,4], k = 2
Output: 5
``````

Example 2:

``````Input: nums = [3,2,3,1,2,4,5,5,6], k = 4
Output: 4
``````

Constraints:

``````1 <= k <= nums.length <= 105
-104 <= nums[i] <= 104
``````

#### SOLUTION:

``````class Solution {
func findKthLargest(_ nums: [Int], _ k: Int) -> Int {
var nums = nums
var start = 0
var end = nums.count-1

var index = nums.count-k
while(start <= end){
var position = partition(&nums, start, end)

if(position == index){
return nums[position]
}else if(position < index){
start = position + 1
}else{
end = position - 1
}
}
return -1
}

func partition(_ nums: inout [Int], _ start: Int, _ end:  Int) -> Int{
var index = start
var pivot = nums[start]
var start = start
var end = end

while(start <= end){

while(start <= end && nums[start] <= pivot){
start += 1
}
while(start <= end && nums[end] >= pivot){
end -= 1
}
if(start <= end){
swap(&nums, start, end)
start += 1
end -= 1
}
}
swap(&nums, index, end)
return end
}

func swap(_ nums: inout [Int], _ start: Int, _ end: Int){
let temp = nums[start]
nums[start]  = nums[end]
nums[end] = temp
}
}
``````