Kth Largest Element in an Array

数组中第k大的数

题目

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.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5
Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4
Note:
You may assume k is always valid, 1 ≤ k ≤ array’s length.

解析重点

1.这一题没有太多可以说的，把排序的代码修改一下就可以了。像堆排、快排等。

java代码

public class Solution {
  
  public int findKthLargest(int[] a, int k) {
    int n = a.length;
    int p = quickSelect(a, 0, n - 1, n - k + 1);
    return a[p];
  }
  
  // return the index of the kth smallest number
  int quickSelect(int[] a, int lo, int hi, int k) {
    // use quick sort's idea
    // put nums that are <= pivot to the left
    // put nums that are  > pivot to the right
    int i = lo, j = hi, pivot = a[hi];
    while (i < j) {
      if (a[i++] > pivot) swap(a, --i, --j);
    }
    swap(a, i, hi);
    
    // count the nums that are <= pivot from lo
    int m = i - lo + 1;
    
    // pivot is the one!
    if (m == k)     return i;
    // pivot is too big, so it must be on the left
    else if (m > k) return quickSelect(a, lo, i - 1, k);
    // pivot is too small, so it must be on the right
    else            return quickSelect(a, i + 1, hi, k - m);
  }
  
  void swap(int[] a, int i, int j) {
    int tmp = a[i];
    a[i] = a[j];
    a[j] = tmp;
  }

}