LeetCode 215 Kth Largest Element in an Array

题目描述

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.

For example,

Given [3,2,1,5,6,4] and k = 2, return 5.

Note:

You may assume k is always valid, 1 ≤ k ≤ array’s length.

Credits:

Special thanks to @mithmatt for adding this problem and creating all test cases.

分析

类似快排，能够达到O(n)的时间复杂度。

代码

[code]    public int findKthLargest(int[] nums, int k) {

        return findK(nums, nums.length - k, 0, nums.length - 1);

    }

    int findK(int[] nums, int k, int low, int high) {

        if (low >= high) {
            return nums[low];
        }

        int p = partition(nums, low, high);

        if (p == k) {
            return nums[p];
        } else if (p < k) {
            // 求第k大的，所以要反过来
            return findK(nums, k, p + 1, high);
        } else {
            return findK(nums, k, low, p - 1);
        }
    }

    int partition(int[] nums, int low, int high) {

        int privotKey = nums[low];

        while (low < high) {

            while (low < high && nums[high] >= privotKey) {
                high--;
            }
            swap(nums, low, high);

            while (low < high && nums[low] <= privotKey) {
                low++;
            }
            swap(nums, low, high);
        }

        return low;
    }

    private void swap(int[] nums, int low, int high) {
        int t = nums[low];
        nums[low] = nums[high];
        nums[high] = t;
    }