杭电oj——1303 Doubles

链接
Problem Description
As part of an arithmetic competency program, your students will be given randomly generated lists of from 2 to 15 unique positive integers and asked to determine how many items in each list are twice some other item in the same list. You will need a program to help you with the grading. This program should be able to scan the lists and output the correct answer for each one. For example, given the list
1 4 3 2 9 7 18 22

your program should answer 3, as 2 is twice 1, 4 is twice 2, and 18 is twice 9.
Input
The input file will consist of one or more lists of numbers. There will be one list of numbers per line. Each list will contain from 2 to 15 unique positive integers. No integer will be larger than 99. Each line will be terminated with the integer 0, which is not considered part of the list. A line with the single number -1 will mark the end of the file. The example input below shows 3 separate lists. Some lists may not contain any doubles.
Output
The output will consist of one line per input list, containing a count of the items that are double some other item.
Sample Input
1 4 3 2 9 7 18 22 0
2 4 8 10 0
7 5 11 13 1 3 0
-1
Sample Output
3
2
0

#include<iostream>
#include<algorithm>
#include<functional> //sort()中greater<int>()所需文件
using namespace std;
int arr[20];//数组在acm竞赛中通常定义为全局变量，防止栈溢出。
int n;
int i,j,s;
int main(){
	while(1){
		cin>>arr[0];
		if(arr[0]==-1){
			break;
		}
		n=1;
		cin>>arr[n];
		while(arr[n]!=0){
			n++;
			cin>>arr[n];
		}
		n--;
		sort(arr,arr+(n+1),greater<int>());//使用sort()进行排序。
		s=0;
		for(i=0;i<n;i++){
			for(j=i+1;j<=n;j++){
				if(arr[i]==arr[j]*2){
					s++;
				}
			}	
		} 
		cout<<s<<endl;
	}
	return 0;
}

枚举
通过先排序再进行枚举降低程序复杂度。