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

 1 #include<cstdio>
 2 #define F(i,a,b) for(int i=a;i<=b;i++)
 3 
 4 int a[1000],n;
 5 void swap(int &a,int &b){int c=a;a=b,b=c;}
 6 
 7 int partition(int *a,int l,int r){
 8     int x=a[l],pos=l;
 9     for(int i=l+1;i<=r;i++)
10         if(a[i]<x)swap(a[++pos],a[i]);
11     swap(a[l],a[pos]);
12     return pos;
13 }
14 
15 int find_k(int *a,int k,int l=1,int r=n){
16     int pos=partition(a,l,r);
17     int j=pos-l+1;
18     if(l==r)return a[l];
19     if(k==j)return a[pos];
20     if(k>j)return find_k(a,k-j,pos+1,r);
21     else return find_k(a,k,l,pos-1);
22 }
23 
24 double findmid(){
25     if(n&1){
26         int k=(n>>1)+1;
27         double ans=find_k(a,k);
28         return ans;
29     }else{
30         int k=n>>1;
31         double ans=find_k(a,k)+find_k(a,k+1);
32         return ans/2;
33     }
34 }
35 
36 int main(){
37     scanf("%d",&n);
38     F(i,1,n)scanf("%d",a+i);
39     printf("mid=%lf\n",findmid());
40     return 0;
41 }

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转载于:https://www.cnblogs.com/bin-gege/p/5696075.html