usaco2.22Subset Sums

看懂題意後 立馬想到dfs 之後隨手寫了一個dfs TLE  看了下人家寫的才知道是揹包

/*
    ID: your_id_here
    PROG: subset
    LANG: C++
*/
#include <iostream>
#include<cstdio>
#include<cstring>
#include<stdlib.h>
#include<algorithm>
using namespace std;
int s,sum,n;
long long f[50][50];
int main()
{
    freopen("subset.in","r",stdin);
    freopen("subset.out","w",stdout);
    int i,j;
    cin>>n;
    s = (n+1)*n/2;
	if(s%2!=0)
	{
		cout<<"0\n";
	}
	else
	{
	    s/=2;
		f[1][0] = 1;
		f[1][1] = 1;
		for(i = 2; i <= n ; i++)
			for(j = s; j >= 0 ; j--)
			{
				if(j>=i)
					f[i][j] = f[i-1][j]+f[i-1][j-i];
				else
					f[i][j] = f[i-1][j];
			}
        cout<<f[n][s]/2<<endl;
	}
    fclose(stdin);
    fclose(stdout);
    return 0;
}