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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
mt19937 rng(std::chrono::system_clock::now().time_since_epoch().count());
const int N = 1e5,M = 998244353;
int n,freq[N],ans = 1;
int power(int base,int to){
	int ret = 1;
	while(to){
		if(to&1)ret = ret*1LL*base%M;
		to>>=1;
		base = base*1LL*base%M;
	}
	return ret;
}
int main(){
	//freopen("input.txt","r",stdin);
	scanf("%d",&n);
	for(int i = 0,a;i < n;i++){
		scanf("%d",&a);
		freq[a]++;
		if(freq[0] != 1)ans = 0;
	}
	for(int i = 1;i < n;i++)ans = ans*1LL*power(freq[i - 1],freq[i])%M;
	printf("%d\n",ans);
}