215. Kth Largest Element in an Array

Posted on One min read

1. Description

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.

2. Example

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

3. Constraints

  • 1 <= k <= nums.length <= 105
  • −104 <= nums[i] <= 104

4. Solutions

Quickselect

n = nums.size()
Time complexity(): O(n)
Space complexity(): O(logn)

class Solution {
public:
    int findKthLargest(vector<int> nums, int k) {
        partition_(nums, 0, nums.size(), k);

        return nums[k - 1];
    }

private:
    void partition_(vector<int> &nums, int left, int right, int k) {
        srand(time(nullptr));
        int random_index = rand() % (right - left) + left;
        int base_number = nums[random_index];
        swap(nums[left], nums[random_index]);
        int index = left;
        for (int i = left + 1; i < right; ++i) {
            if (nums[i] > base_number) {
                swap(nums[i], nums[index + 1]);
                ++index;
            }
        }

        swap(nums[left], nums[index]);

        if (index < k - 1) {
            partition_(nums, index + 1, right, k);
        } else if (index > k - 1) {
            partition_(nums, left, index, k);
        }
    }
};