LeetCode-215.Kth Largest Element in an Array

https://leetcode.com/problems/kth-largest-element-in-an-array/

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.

1 

public int FindKthLargest(int[] nums, int k)     {        Array.Sort(nums);        return nums[nums.Length-k];    }

2

使用基于红黑树实现的multiset数据结构

int findKthLargest(vector<int>& nums, int k)     {    multiset<int> res;    for (int num : nums)    {    if (res.size() < k)    res.insert(num);    else    {    if (num > *res.begin())    {    res.erase(res.begin());    res.insert(num);    }    }    }    return *res.begin();    }

3

快排

public class Solution{    public int FindKthLargest(int[] nums, int k)     {        Sort(nums, 0, nums.Length - 1);        return nums[k-1];    }        private void Sort(int[] nums, int l, int r)    {        if (l<r)        {            int i = l,j=r,x=nums[i];            while (i<j)            {                while (i < j && nums[j] <= x)                    j--;                if (i < j)                {                    nums[i] = nums[j];                    i++;                }                while (i < j && nums[i] > x)                    i++;                if (i < j)                {                    nums[j] = nums[i];                    j--;                }             }            nums[i] = x;            Sort(nums, l, i - 1);            Sort(nums, i + 1, r);        }    }}

针对此题优化排序函数，

因为只要第k大，所以不用全排序

private void Sort(int[] nums, int l, int r,int k)    {        if (l<r)        {            int i = l,j=r,x=nums[i];            while (i<j)            {                while (i < j && nums[j] <= x)                    j--;                if (i < j)                {                    nums[i] = nums[j];                    i++;                }                while (i < j && nums[i] > x)                    i++;                if (i < j)                {                    nums[j] = nums[i];                    j--;                }             }            nums[i] = x;            if (i+1 == k)                return;            else if(i+1>k)                Sort(nums, l, i - 1,k);            else                Sort(nums, i + 1, r,k);        }    }