2017上海金马五校程序设计竞赛 C ：Count the Number

Time Limit: 3 s

Description

Given n numbers, your task is to insert '+' or '-' in front of each number to construct expressions. Note that the position of numbers can be also changed.

You can calculate a result for each expression. Please count the number of distinct results and output it.

 Input

There are several cases.

For each test case, the first line contains an integer n (1 ≤ n ≤ 20), and the second line contains n integers a₁,a₂, ... ,a_n(-1,000,000,000 ≤ a_i ≤ 1,000,000,000).

Output

For each test case, output one line with the number of distinct results

Sample Input

21 231 3 5

 Sample Output

4
8

本题题意：

这题说的是第一行给你一个n,接下来一行给你n个数，可以在这n 个数前边加“+”或者“-”，计算这些数的和，输出有几种不同的和。

解题思路：

因为数据比较小，直接暴力深搜就可以。

代码：

#include <iostream>#include <algorithm>#include <map>#include <cstdio>using namespace std;int a[25];map <int,int> M;int n,ans;void dfs(int sum,int i){    if(i==n+1)    {        if(M[sum]==0)///如果没有出现过        {            M[sum]=1;            ans++;        }        return;    }    dfs(sum+a[i],i+1);    dfs(sum-a[i],i+1);}int main(){    while(~scanf("%d",&n))    {        for(int i=0; i<n; i++)            scanf("%d",&a[i]);        M.clear();        ans=0;        dfs(0,0);        printf("%d\n",ans);    }    return 0;}

Output