LeetCode 215. Kth Largest Element in an Array

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本文介绍了解决LeetCode 215题“数组中的第K个最大元素”的快速选择算法。该算法通过一次遍历找到第K大的元素，并达到平均线性的时间复杂度。文中提供了一个Java实现示例。

LeetCode 215. Kth Largest Element in an Array

Description

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note:

You may assume k is always valid, 1 ≤ k ≤ array’s length.


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

        private int quickSelect(int[] nums, int lo, int hi, int k) {
            int lt = lo;
            int gt = hi + 1;
            int i = lo + 1;
            int pivot = nums[lo];

            while (i < gt) {
                if (nums[i] > pivot) {
                    swap(nums, i, lt + 1);
                    lt++;
                    i++;
                }
                else if (nums[i] < pivot) {
                    swap(nums, i, gt - 1);
                    gt--;
                }
                else {
                    i++;
                }
            }

            swap(nums, lo, lt);

            if (lt == k) {
                return lt;
            }
            else if (lt < k) {
                return quickSelect(nums, lt + 1, hi, k);
            }
            else {
                return quickSelect(nums, lo, lt - 1, k);
            }
        }


        private void swap(int[] a, int i, int j) {
            int temp = a[i];
            a[i] = a[j];
            a[j] = temp;
        }
    }