[lintcode]Kth Largest Element

寻找第K大元素

最新推荐文章于 2024-04-15 15:35:54 发布

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最新推荐文章于 2024-04-15 15:35:54 发布

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原文链接：http://www.cnblogs.com/kittyamin/p/4996498.html

本文介绍了一种在数组中寻找第K大元素的有效算法。通过快速选择算法实现，该算法能够在O(n)时间内找到目标元素，并且仅使用O(1)额外内存。文章提供了详细的实现代码，展示了如何通过不断划分数组来定位目标元素。

Kth Largest Element

Find K-th largest element in an array.

Example

In array [9,3,2,4,8], the 3rd largest element is 4.

In array [1,2,3,4,5], the 1st largest element is 5, 2nd largest element is 4, 3rd largest element is 3 and etc.

Note

You can swap elements in the array

Challenge

O(n) time, O(1) extra memory.

class Solution {
    //param k : description of k
    //param numbers : array of numbers
    //return: description of return
    public int kthLargestElement(int k, ArrayList<Integer> numbers) {
        // write your code here
         int L = 0, R = numbers.size() - 1;
         while (L < R) {
             int left = L, right = R;
             int key = numbers.get(left);
             while (left < right) {
                 while (left<right &&numbers.get(right) < key) --right;
                 numbers.set(left,numbers.get(right));
                 while (left<right &&numbers.get(left)>= key) ++left;
                 numbers.set(right,numbers.get(left)); 
             }
             numbers.set(left,key);
             if (left == k - 1) return numbers.get(k-1);
             else if (left > k - 1) R = left - 1;
             else L = left + 1;
        }
        return numbers.get(k-1);
   }
};