Acwing-----基础课复习

Posted 2020-11-27 clown9804

第一讲：基础算法

785. 快速排序

• 链接：https://www.acwing.com/problem/content/787/

#include <iostream>
using namespace std;
const int N = 1000010;
int q[N];

void quick_sort(int q[], int l, int r) {
    if (l >= r) return ;
    int x = q[(l + r) / 2], i = l - 1, j = r + 1;
    while (i < j) {
        do i++;while (q[i] < x);
        do j--;while (q[j] > x);
        if (i < j) swap(q[i], q[j]);
    }
    quick_sort(q, l, j);
    quick_sort(q, j + 1, r);
}

int main() {
    int n;
    cin >> n;
    
    for (int i = 0; i < n; ++i) cin >> q[i];
    quick_sort(q, 0, n - 1);
    for (int i = 0; i < n; ++i) cout << q[i] << " ";
    puts("");
    
    return 0;
}

786. 第k个数

• 链接：https://www.acwing.com/problem/content/788/

#include <iostream>
using namespace std;

const int N = 100010;
int n, k, q[N];

int quick_sort(int l, int r, int k) {
    if (l >= r) return q[l];
    
    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    
    while (i < j) {
        do i ++ ; while (q[i] < x);
        do j -- ; while (q[j] > x);
        if (i < j) swap(q[i], q[j]);
    }
    
    int sl = j - l + 1;
    if (k <= sl) return quick_sort(l, j, k);
    return quick_sort(j + 1, r, k - sl);
}

int main() {
    scanf("%d%d", &n, &k);
    for (int i = 0; i < n; ++i) scanf("%d", &q[i]);
    cout << quick_sort(0, n - 1, k) << endl;
}