ZSOI2012 這是一棵樹嗎？ 姑且算圖論?

其實隨便判判就好了，不用判是否有入度爲1的，證明比較簡單

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#define LL long long
#define fo(i,a,b) for(int i=a;i<=b;i++)
#define fd(i,a,b) for(int i=a;i>=b;i--)
using namespace std;
inline LL read()
{
	LL d=0,f=1;char s=getchar();
	while(s<'0'||s>'9'){if(s=='-')f=-1;s=getchar();}
	while(s>='0'&&s<='9'){d=d*10+s-'0';s=getchar();}
	return d*f;
}
#define N 105
int du[N];
int n;
int sum=0;

bool com(int a,int b)
{
	return a>b;
}

int main()
{
	freopen("tree.in","r",stdin);
	freopen("tree.out","w",stdout);
	bool flag=0;
	n=read();
	fo(i,1,n)
	{
		du[i]=read();
		if(n>1&&du[i]==0)
		{
			puts("Impossible");
			return 0;
		}
		if(n>1&&du[i]==1)flag=1;
		sum+=du[i];
	}
	if(sum%2||(sum/2)!=n-1||(!flag&&n>1))
	{
		puts("Impossible");
		return 0;
	}
	puts("Possible");
	return 0;
}