ZSOI2012 这是一棵树吗？ 姑且算图论?

其实随便判判就好了，不用判是否有入度为1的，证明比较简单

#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
;
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;
}