找包含N个元素的数组里第K大的元素（引申：快速排序、找中位数、找前K大的元素）的时间复杂度

#include<iostream>using namespace std;void swap(int &a, int &b){int temp = a;a = b;b = temp;}int partition(int a[],int begin,int end){int low,high,key;low = begin;high = end;key = a[low];while(low < high){while(low < high && a[high] >= key) //注意要取等号，否则当全部待排序数字都相同的时候会死循环！high--;swap(a[low],a[high]);while(low < high && a[low] <= key ) //pay attention to the equal, otherwise it will get into endless loop!low++;swap(a[low],a[high]);}a[low] = key;return low;}void qsort(int a[], int begin, int end){int p = partition(a,begin,end);if(p > begin)qsort(a, begin,p-1);if(p < end)qsort(a,p+1,end);}int findK(int a[],int begin, int end,int k){if(!(k>=begin && k<= end))return 0;int p = partition(a,begin,end);if(p > k)findK(a,begin,p-1,k);else if(p < k)findK(a,p+1,end,k);else if(p == k)return a[p];}int main(){int a[] = {5,2,3,4,6,8},i;int t = partition(a,0,5);cout<<t<<endl;for(i =0; i<6;i++)cout<<a[i]<<' ';cout<<endl; qsort(a,0,5);cout<<"After sorted: "<<endl;for(i =0; i<6;i++)cout<<a[i]<<' ';cout<<endl;int k = findK(a,0,5,5);cout<<k<<endl;return 0; }