Problem:
Score : 300 points

Problem Statement

Given is an integer sequence D_1,...,D_N of N elements. Find the number, modulo 998244353, of trees with N vertices numbered 1 to N that satisfy the following condition:

For every integer i from 1 to N, the distance between Vertex 1 and Vertex i is D_i.

Notes

A tree of N vertices is a connected undirected graph with N vertices and N-1 edges, and the distance between two vertices are the number of edges in the shortest path between them.

Two trees are considered different if and only if there are two vertices x and y such that there is an edge between x and y in one of those trees and not in the other.

Constraints

1 \leq N \leq 10^5

0 \leq D_i \leq N-1

Input

Input is given from Standard Input in the following format:

N
D_1 D_2 ... D_N

Output

Print the answer.

Sample Input 1

4
0 1 1 2

Sample Output 1

2

For example, a tree with edges (1,2), (1,3), and (2,4) satisfies the condition.

Sample Input 2

4
1 1 1 1

Sample Output 2

0

Sample Input 3

7
0 3 2 1 2 2 1

Sample Output 3

24

Solution in Python (3.4.3) (judge verdict: Accepted)

import sys
import collections

n=int(input())
d = list(map(int, input().split()))

mod = 998244353
	
c = collections.Counter(d)
c = sorted(c.items(), key=lambda x:x[0])

if c==[(0,1)]:
	print(1)
	sys.exit()

if d[0]!=0 or c[0]!=(0,1):
	print(0)
	sys.exit()

ans=1
for i in range(1,len(c)):	
	ans = ans*(c[i-1][1])**c[i][1]
	if c[i-1][0]+1!=c[i][0]:
		print(0)
		sys.exit()

print(ans%mod)