O(n)线性时间找第K大，中位数

运用快速排序的思想，可以达到线性时间找到一串数的第K大

#include<cstdio>
#define F(i,a,b) for(int i=a;i<=b;i++)

int a[1000],n;
void swap(int &a,int &b){int c=a;a=b,b=c;}

int partition(int *a,int l,int r){
    int x=a[l],pos=l;
    for(int i=l+1;i<=r;i++)
        if(a[i]<x)swap(a[++pos],a[i]);
    swap(a[l],a[pos]);
    return pos;
}

int find_k(int *a,int k,int l=1,int r=n){
    int pos=partition(a,l,r);
    int j=pos-l+1;
    if(l==r)return a[l];
    if(k==j)return a[pos];
    if(k>j)return find_k(a,k-j,pos+1,r);
    else return find_k(a,k,l,pos-1);
}

double findmid(){
    if(n&1){
        int k=(n>>1)+1;
        double ans=find_k(a,k);
        return ans;
    }else{
        int k=n>>1;
        double ans=find_k(a,k)+find_k(a,k+1);
        return ans/2;
    }
}

int main(){
    scanf("%d",&n);
    F(i,1,n)scanf("%d",a+i);
    printf("mid=%lf
",findmid());
    return 0;
}