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 (GCC 9.2.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <stdbool.h>
typedef long long LL;
typedef long double Lf;
#define Rep(i,a,n)for(LL i=(LL)(a);i<(LL)(n);i++)
#define rep(i,n)Rep(i,0,n)
#define Repp(i,l,r,k)for(LL i=(l);i<(r);i+=(k))
#define Ma(x,y) ((x)>(y)?(x):(y))
#define mi(x,y) ((x)<(y)?(x):(y))
#define MOD 998244353//1000000007,1000000009,998244353 
int upi(const void* a, const void* b) { return*(int*)a<*(int*)b ? -1 : *(int*)a> * (int*)b ? 1 : 0; }
int downi(const void* a, const void* b) { return*(int*)a<*(int*)b ? 1 : *(int*)a> * (int*)b ? -1 : 0; }
void sortupi(int* a, int n) { qsort(a, n, sizeof(int), upi); } //1,2,3,4,...
void sortdowni(int* a, int n) { qsort(a, n, sizeof(int), downi); }
int upLL(const void* a, const void* b) { return*(LL*)a<*(LL*)b ? -1 : *(LL*)a> * (LL*)b ? 1 : 0; }
int downLL(const void* a, const void* b) { return*(LL*)a<*(LL*)b ? 1 : *(LL*)a> * (LL*)b ? -1 : 0; }
void sortupLL(LL* a, int n) { qsort(a, n, sizeof(LL), upLL); }
void sortdownLL(LL* a, int n) { qsort(a, n, sizeof(LL), downLL); }
int cmp_char(const void* a, const void* b) { return *(char*)a - *(char*)b; } // a,b,c,d
int cmp_str(const void* a, const void* b) { return strcmp(*(const char**)a, *(const char**)b); } // aaa,aab
LL gcd(LL p, LL q) { while (q) { LL t = p % q; p = q; q = t; }return p; }
LL modpow(LL a, LL n, int m) { LL x = 1; for (a %= m; n; n /= 2)n & 1 ? x = x * a % m : 0, a = a * a % m; return x; }
#define invmodpow(b,p)modpow(b,p-2,p)// a/b≡a*b^{-1}(mod p)...return b^{-1}(mod p)
typedef struct fac { LL n, exp; }F;
LL fact(LL n,F*a){LL x,i,j,c=0;x=n;for(i=2;i*i<=x;i++){for(j=1;x%i==0;j++)x/=i;if(j>1){a[c].n=i;a[c].exp=j-1;c++;}}if(x-1){a[c].n=x;a[c].exp=1;c++;}return c;}

//snippet
#define N 100009

int main(){
  int n,m=0,a[N],b[N]={0};LL ans;
  scanf("%d",&n);
  rep(i,n){
    scanf("%d",a+i);
    b[a[i]]++;
    m=mi(m,a[i]);
  }
  ans=(a[0]||b[0]-1?0:1);
  rep(i,n-1)ans=(ans*b[a[i+1]-1])%MOD;
  printf("%lld",ans);
}