leetcode 215. 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.

利用快排的partition函数思想，选定一个数组内的值作为pivot，将小于pivot的数字放到pivot右边，大于等于pivot的数字放到pivot左边。接着判断两边数字的数量，如果左边的数量小于k个，说明第k大的数字存在于pivot及pivot右边的区域之内，对右半区执行partition函数；如果右边的数量小于k个，说明第k大的数字在pivot和pivot左边的区域之内，对左半区执行partition函数。直到左半区刚好有k-1个数，那么第k大的数就已经找到了

class Solution {public:    int findKthLargest(vector<int>& nums, int k)     {        int high = nums.size();        int low = 0;        while (low < high)         {            int i = low;            int j = high-1;            int pivot = nums[low];            while (i <= j) {                while (i <= j && nums[i] >= pivot)                    i++;                while (i <= j && nums[j] < pivot)                    j--;                if (i < j)                    swap(nums[i++],nums[j--]);            }            swap(nums[low],nums[j]);            if (j == k-1)                return nums[j];            else if (j < k-1)                low = j+1;            else                high = j;        }     }};