[email protected]_215_Kth_Largest_Element_in_an_Array

Problem:

Algorithm

算法导论：Order Statistic

Java：

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;
  }

}