堆排序

 

/**
 * PriorityQueue 原理：最小堆
 * peek获取且不删除堆顶元素，poll获取且删除堆顶元素
 *
 * 215. 数组中的第K个最大元素
 * https://leetcode-cn.com/problems/kth-largest-element-in-an-array/
 */
public class FindKthLargest {

    public int findKthLargest(int[] nums, int k) {
        buildMaxHeap(nums);
        int length = nums.length;
        for (int i = 0; i < k - 1; i++) {
            swap(nums, 0, length - 1 - i);
            heapify(nums, 0, length - 1 - i);
        }
        return nums[0];
    }

    public void buildMaxHeap(int[] nums) {
        int length = nums.length;
        for (int i = length / 2 - 1; i >= 0; i--) {
            heapify(nums, i, length);
        }
    }

    public void heapify(int[] nums, int headIndex, int length) {
        int lagestIndex = headIndex, leftIndex = headIndex * 2 + 1, rightIndex = headIndex * 2 + 2;
        if (leftIndex < length && nums[leftIndex] > nums[lagestIndex]) {
            lagestIndex = leftIndex;
        }
        if (rightIndex < length && nums[rightIndex] > nums[lagestIndex]) {
            lagestIndex = rightIndex;
        }
        if (lagestIndex != headIndex) {
            swap(nums, lagestIndex, headIndex);
            heapify(nums, lagestIndex, length);
        }
    }

    public void swap(int[] nums, int lIndex, int rIndex) {
        int temp = nums[lIndex];
        nums[lIndex] = nums[rIndex];
        nums[rIndex] = temp;
    }

}