Kth Largest Element in an Array LeetCode Solution

Problem – Kth Largest Element in an Array

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

Kth Largest Element in an Array LeetCode Solution in C++

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        buildMaxHeap(nums);
        for (int i = 0; i < k - 1; i++) {
            swap(nums[0], nums[--heapSize]);
            maxHeapify(nums, 0);
        }
        return nums[0];
    }
private:
    int heapSize;
    
    int left(int i) {
        return (i << 1) + 1;
    }
    
    int right(int i) {
        return (i << 1) + 2;
    }
    
    void maxHeapify(vector<int>& nums, int i) {
        int largest = i, l = left(i), r = right(i);
        if (l < heapSize && nums[l] > nums[largest]) {
            largest = l;
        }
        if (r < heapSize && nums[r] > nums[largest]) {
            largest = r;
        }
        if (largest != i) {
            swap(nums[i], nums[largest]);
            maxHeapify(nums, largest);
        }
    }
    
    void buildMaxHeap(vector<int>& nums) {
        heapSize = nums.size();
        for (int i = (heapSize >> 1) - 1; i >= 0; i--) {
            maxHeapify(nums, i);
        }
    }
};

Kth Largest Element in an Array LeetCode Solution in Python

class Solution:
    def findKthLargest(self, nums, k):
        if not nums: return
        pivot = random.choice(nums)
        left =  [x for x in nums if x > pivot]
        mid  =  [x for x in nums if x == pivot]
        right = [x for x in nums if x < pivot]
        
        L, M = len(left), len(mid)
        
        if k <= L:
            return self.findKthLargest(left, k)
        elif k > L + M:
            return self.findKthLargest(right, k - L - M)
        else:
            return mid[0]

Kth Largest Element in an Array LeetCode Solution in Java

public int findKthLargest(int[] nums, int k) {
  return quickSelect(nums, 0, nums.length - 1, k);
}

int quickSelect(int[] nums, int low, int high, int k) {
  int pivot = low;

  // use quick sort's idea
  // put nums that are <= pivot to the left
  // put nums that are  > pivot to the right
  for (int j = low; j < high; j++) {
    if (nums[j] <= nums[high]) {
      swap(nums, pivot++, j);
    }
  }
  swap(nums, pivot, high);
  
  // count the nums that are > pivot from high
  int count = high - pivot + 1;
  // pivot is the one!
  if (count == k) return nums[pivot];
  // pivot is too small, so it must be on the right
  if (count > k) return quickSelect(nums, pivot + 1, high, k);
  // pivot is too big, so it must be on the left
  return quickSelect(nums, low, pivot - 1, k - count);
}
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