选择问题（分治策略）

 1 //选择问题
 2 #include<iostream>
 3 #include<cstdio>
 4 #include<cstdlib>
 5 #include<algorithm>
 6 using namespace std;
 7 int a[1001];
 8 
 9 int select(int left,int right,int k){
10     if( left >= right )
11         return a[left];
12     int x = a[left];
13     int i = left;
14     int j = right+1;
15     while( true ){
16         do{
17             i++;
18         }while(a[i]<x);
19         do{
20             j--;
21         }while(a[j]>x);
22         if( i>=j )
23             break ;
24         swap(a[i],a[j]);
25     }
26     if( j-left+1 == k )
27         return x;
28     a[left] = a[j];
29     a[j] = x;
30     if( j-left+1 < k )
31         return select(j+1,right,k-j+left-1);
32     else
33         return select(left,j-1,k); 
34 }
35 
36 int main(){
37     int n;
38     while( scanf("%d",&n) != EOF ){
39         int k;
40         cin>>k;
41         for( int i = 0; i < n; i++ )
42             cin>>a[i];
43         cout<<select(0,n-1,k)<<endl;
44     }
45     return 0;
46 }