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: Wrong Answer)

N = int(input())
D = list(map(int, input().strip().split()))
D.sort()
# print(D)
 
def calc(arr):
  if arr[0] != 0:
    return 0
  result = 1
  curCount = 1
  preCount = 1
  for i in range(1, len(arr)):
    if arr[i] == 0:
      return 0
    if arr[i] == arr[i-1]:
      curCount += 1
    elif arr[i] == arr[i-1] + 1:
      preCount = curCount
      curCount = 1
    else:
      return 0
    result = result * preCount
    if result > 998244353:
      result = result % 998244353
    #print(pre, curCount, preCount, result)
  return result%998244353
 
print(calc(D))