LeetCode 215. Kth Largest Element in an Array

Description

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.

Solution

将数组排序，然后直接通过索引返回第k大的数字

int findKthLargest(vector<int>& nums, int k){    sort(nums.begin(), nums.end());    reverse(nums.begin(), nums.end());    return nums[k-1];}

利用分治思想写出快速选择算法耶鲁大学：QuickSelect算法

#include <iostream>#include <vector>using namespace std;int findKthLargest(vector<int>& nums, int k){    vector<int> left;    vector<int> right;    int pivot = nums[0];    for(int i = 1; i < nums.size(); i++){        if(nums[i] <= pivot) left.push_back(nums[i]);        else right.push_back((nums[i]));    }    if(k <= right.size()) return findKthLargest(right, k);    else if(k-1 == right.size()) return pivot;    else return findKthLargest(left, k-right.size()-1);}int main(){    int ary[10] = {3, 2, 1, 5, 6, 4};    vector<int> vec(ary, ary+6);    cout<<findKthLargest(vec, 2);}

快速选择算法在LeetCode平台上会出现内存超限（Memory Limit Exceeded）的问题，这与C++中内存的管理模式有关：vector内存空间只会增长，不会减小。