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)

from collections import Counter
n = int(input())
e = list(map(int, input().split()))

if e[0] != 0:
  ans = 0
else:
  ce = Counter(e)
  sce = sorted(ce.items())
  # keyの数が連番であることを確認
  if sce[-1][0] != len(sce) - 1 or sce[0][1] != 1:
    ans = 0
  else:
    ans = 1
    pre = 1
    for i, j in sce:
      if pre != 1:
        ans = (ans * (pre ** j)) % 998244353
      pre = j
      
print(ans)