快速排序

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<ctime>
#include<iostream>
#define maxn 21111111
using namespace std;
int n, a[maxn];
void sort(int ll, int rr)
{
    if (ll >= rr) return;
    int x = a[ll+rand()%(rr-ll+1)];
    int i = ll, j = rr,p=ll,q=rr;
    while (i <= j)
    {
        while (a[i] < x) i++;
        while (x < a[j]) j--;
        if (i <= j)
        {
            swap(a[i], a[j]);
            if (a[i] == x) swap(a[p++], a[i]); 
            if (i!=j && a[j] == x) swap(a[q--], a[j]); //i!=j重要 因为只要把相等的归在一边就可以了（左边）
            i++;
            j--;
        }
    }
    j = i - 1; //注意非常重要 因为有可能循环结束前i=j     j __ i  所以要强行设置j=i-1
    for (int k = ll; k < p; k++, j--) swap(a[k], a[j]);
    for (int k = rr; k>q; k--, i++) swap(a[k], a[i]);
    sort(ll, j);
    sort(i, rr);
}
int main()
{
    freopen("1.in","r",stdin);
    freopen("1.out","w",stdout);
    srand(time(0));
    scanf("%d", &n);
    for (int i = 0; i < n; i++) scanf("%d", &a[i]);
    sort(0, n - 1);
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    printf("\n");
    return 0;
}

#include<ctime>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<iostream>
#define maxn 21111111
using namespace std;
int n,a[maxn];
void sort(int ll, int rr)
{
    if (ll == rr) return;
    int key = a[rand() % (rr - ll + 1) + ll];
    int i = ll,j = rr;
    while (i <= j)
    {
        while (a[i] < key) i++;
        while (a[j] > key) j--;
        if (i <= j) { swap(a[i], a[j]); i++; j--; }
    }
    if (ll < j) sort(ll, j);
    if (i < rr) sort(i, rr);
}
int main()
{
    freopen("1.in","r",stdin);
    freopen("1.out","w",stdout);
    srand(time(0)); 
    scanf("%d", &n);
    for (int i = 0; i < n; i++) scanf("%d", &a[i]);
    sort(0, n - 1);
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    return 0;
}