leetcode之 Kth Largest Element in an Array

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.

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

解答:

注意题目中写的是第k大不是第k小

思想很简答，就是利用快速排序的思想来实现，之前在算法设计分析上学习过， 总体复杂度是O(n)的（但是忘记怎么算出这个复杂度的了，想要看的可以看书）

同时还注意到C++中已经有了直接找到第n个数的函数，nth_element(),想要知道用法的可以自己问度娘

class Solution {public:    void swap(int i,int j, vector<int>& nums)    {        int t = nums[i];        nums[i] = nums[j];        nums[j] = t;    }    int kth(vector<int>& nums, int i, int j, int k)    {        while(true)        {        int pos1 = i, pos2 = i;        int tag = nums[j];        while(pos2 < j)        {            if(nums[pos2] < tag)            {                swap(pos1,pos2,nums);                pos1++;                pos2++;            }            else                pos2++;        }        if(pos1 - i + 1 == k)            return tag;        if(pos1 - i + 1 > k)        {            j = pos1 - 1;        }        if(pos1 - i + 1 < k)        {            i = pos1;            j = j - 1;            k = k - pos1 - 1 + i;        }        }    }    int findKthLargest(vector<int>& nums, int k) {        return kth(nums, 0, nums.size() - 1, nums.size() - k + 1);    }};