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)

n = int(input())
dl = list(map(int,input().split()))
if dl[0] != 0:
  print(0)
  exit()
d_dic = dict()
for d in dl:
  if d in d_dic:
    d_dic[d] += 1
  else:
    d_dic[d] = 1
l = max(d_dic.keys())
for i in range(l):
  if not i in d_dic:
    print(0)
    exit()
if d_dic[0] != 1:
  print(0)
  exit()
ans = 1
for i in range(l):
  ans *= d_dic[i]**d_dic[i+1]
  ans = ans%998244353
print(ans)