【结论？】这是一颗树吗？

$Link$Link

$luogu\ T145300$luogu T145300

$Description$Description

$Sample$Sample $Input$Input

1

1 0

2

2 1 1

3

3 2 2 2

4

3 1 2 1

$Sample$Sample $Output$Output

1

Possible

2

Possible

3

Impossible

4

Possible

$Hint$Hint

$Solution$Solution

就。。推着推着就发现一个结论？？？
$sum = (n - 1 )* 2$sum=(n−1)∗2???
然后特判1啥的就没了

$Code$Code

#include<iostream>
#include<cstdio>
#include<cmath>

using namespace std;

int n, sum, a[105], ans; 

int main()
{
	scanf("%d", &n);
	if (n == 1) {
		int x; 
		scanf("%d", &x);
		if (!x) printf("Possible");
		else printf("Impossible");
		return 0;
	}
	else {
		for (int i = 1; i <= n; ++i)
		{
			scanf("%d", &a[i]);
			if (a[i] <= 0) {
				printf("Impossible");
				return 0;
			}//判负
			if (a[i] == n) {
				printf("Impossible");
				return 0;
			} 
			ans += a[i];
		}
		if (ans / 2 == n - 1) printf("Possible");
		else printf("Impossible");
	}
	return 0;
}