215. Kth Largest Element in an Array

最新推荐文章于 2022-07-02 13:48:51 发布

nemoyy

最新推荐文章于 2022-07-02 13:48:51 发布

阅读量192

点赞数

本文链接：https://blog.youkuaiyun.com/nemoyy/article/details/60478801

版权

本文介绍了一种高效算法，用于从无序数组中找到第K大的元素。通过一次快速选择将前K大元素置于数组右侧，并对这K个元素进行快速排序来实现。这种方法结合了快速选择与快速排序的优点。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

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.

class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        arrangeRight(nums,0,nums.size()-1,k);//将前k个大数放在nums右边，O(n).
        quickSort(nums,nums.size()-k,nums.size()-1); //对前k个数快排。O(klogk).
        return nums[nums.size()-k];
    }

    void arrangeRight(vector<int>& nums, int start, int end, int k){
    	if(start >= end)
    		return;
    	int key = nums[start];
    	int i = start, j = end;
    	while(i!=j){
    		while(j>i && nums[j] >= key)
    			--j;
    		swap(nums,i,j);
    		while( i<j && nums[i] <= key)
    			++i;
    		swap(nums,i,j);
    	}//a[i] = key
    	if(k == (nums.size() - i)) return;
    	if(k < (nums.size() - i)) arrangeRight(nums,i+1,end,k);
    	if(k > (nums.size() - i)) arrangeRight(nums,start,i-1,k-(nums.size()-i));
    }
    
    void quickSort(vector<int>& nums, int start, int end){
    	if(start >= end)
    		return;
    	int key = nums[start];
    	int i = start, j = end;
    	while(i!=j){
    		while(j>i && nums[j] >= key)
    			--j;
    		swap(nums,i,j);
    		while( i<j && nums[i] <= key)
    			++i;
    		swap(nums,i,j);
    	}
    	quickSort(nums,start,i-1);
    	quickSort(nums,i+1,end);
    }
    void swap(vector<int> &v,int a,int b){
        int t = v[a];
        v[a] = v[b];
        v[b] = t;
    } 
};

Solution explained: 点击打开链接