快速选择法
#include <iostream>
using namespace std;
const int N = 1e5 + 10;
int n, k;
int q[N];
int quick_sort(int l, int r, int k)
{
if (l == r) return q[l];
int i = l - 1, j = r + 1, mid = q[l + r >> 1];
while (i < j)
{
while (q[ ++ i] < mid);
while (q[ -- j] > mid);
if (i < j) swap(q[i], q[j]);
}
if (k <= j - l + 1) return quick_sort(l, j, k);
return quick_sort(j + 1, r, k - (j - l + 1)); //j - l + 1 为二分后前一段的长度
}
int main()
{
cin >> n >> k;
for (int i = 0; i < n; i ++ ) cin >> q[i];
cout << quick_sort(0, n - 1, k) << endl;
return 0;
}
快速排序法
#include <iostream>
using namespace std;
const int N = 1e6 + 10;
int q[N];
int quick_sort(int q[], int l, int r) {
if (l >= r) return 0;
int mid = q[l + r >> 1], i = l - 1, j = r + 1;
while (i < j) {
do i++; while (q[i] < mid);
do j--; while (q[j] > mid);
if (i < j) swap(q[i], q[j]);
}
quick_sort(q, l, j);
quick_sort(q, j + 1, r);
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, k;
cin >> n >> k;
for (int i = 0; i < n; i ++ ) cin >> q[i];
quick_sort(q, 0, n - 1);
printf("%d\n", q[k - 1]);
return 0;
}