215. Kth Largest Element in an Array

问题描述：

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.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5

Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4

Note: 
You may assume k is always valid, 1 ≤ k ≤ array's length.

解题思路：

这很容易想到，可以对数组进行排序，然后直接取k个最大的数字。

这样的时间复杂度为O(nlogn)

而使用partition来进行分割，可以达到均摊时间O(n)的时间复杂度。

代码：

class Solution {
public:
    int partition(vector<int> &nums, int left, int right){
        int l = left, r = right;
        int pivot = nums[left];
        while(l < r){
            while(nums[r] <= pivot && r > l) r--;
            nums[l] = nums[r];
            while(nums[l] > pivot && r > l) l++;
            nums[r] = nums[l];
        }
        nums[l] = pivot;
        return r;
    }
    int findKthLargest(vector<int>& nums, int k) {
        int p = -1, l = 0, r = nums.size()-1;
        while(true){
            p = partition(nums, l, r);
            if(p == k-1) break;
            else if(p > k-1) r = p - 1;
            else l = p + 1;
        }
        return nums[p];
    }
};