215. 數組中的第K個最大元素（排序）（快速選擇排序[複習]）

方法一：排序，計數器

class Solution {
    public int findKthLargest(int[] nums, int k) {
        Arrays.sort(nums);

        return nums[nums.length-k];
    }
}

方法二： 用堆
最小堆 + 維護一個只有k個元素；

class Solution {
    public int findKthLargest(int[] nums, int k) {
		//創建堆，注意這裏應用了lambda表達式
        PriorityQueue<Integer> heap = new PriorityQueue<>((n1,n2) ->(n1-n2));
        for(int i :nums){
            heap.add(i);
            //維持當前堆的大小保持在k個元素
            if(heap.size()>k)
                heap.poll();
        }

        return heap.poll();
    }
}

方法三： 快速選擇排序算法（注意不是快速排序，是基於快排修改得來的）

import java.util.Random;
class Solution {
  int [] nums;

  public void swap(int a, int b) {
    int tmp = this.nums[a];
    this.nums[a] = this.nums[b];
    this.nums[b] = tmp;
  }


	//劃分
  public int partition(int left, int right, int pivot_index) {
    int pivot = this.nums[pivot_index];
    // 1. move pivot to end
    swap(pivot_index, right);
    int store_index = left;

    // 2. move all smaller elements to the left
    for (int i = left; i <= right; i++) {
      if (this.nums[i] < pivot) {
        swap(store_index, i);
        store_index++;
      }
    }

    // 3. move pivot to its final place
    swap(store_index, right);

    return store_index;
  }

  public int quickselect(int left, int right, int k_smallest) {
    /*
    Returns the k-th smallest element of list within left..right.
    */

    if (left == right) // If the list contains only one element,
      return this.nums[left];  // return that element

    // select a random pivot_index
    Random random_num = new Random();
    int pivot_index = left + random_num.nextInt(right - left); 
    
    pivot_index = partition(left, right, pivot_index);

    // the pivot is on (N - k)th smallest position
    if (k_smallest == pivot_index)
      return this.nums[k_smallest];
    // go left side
    else if (k_smallest < pivot_index)
      return quickselect(left, pivot_index - 1, k_smallest);
    // go right side
    return quickselect(pivot_index + 1, right, k_smallest);
  }

  public int findKthLargest(int[] nums, int k) {
    this.nums = nums;
    int size = nums.length;
    // kth largest is (N - k)th smallest
    return quickselect(0, size - 1, size - k);
  }
}