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: Wrong Answer)

#include <iostream>
#include <vector>

#define int long long int

using namespace std;

const int mod = 998244353;

int pow_mod(int a, int p)
{
    int cur = a, ans = 1;

    while (p > 0)
    {
        if (p & 1)
            ans = (ans * cur) % mod;
        p /= 2;
        cur = (cur * cur) % mod;
    }

    return ans;
}

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    int n;
    cin >> n;

    vector<int> d(n);
    vector<int> cnt(n, 0);
    for (int i = 0; i < n; i++)
    {
        cin >> d[i];
        cnt[d[i]]++;
    }

    if (cnt[0] != 1)
    {
        cout << "0\n";
        return 0;
    }

    int res = 1;
    for (int i = 1; i < n; i++)
        res = (res * pow_mod(cnt[i - 1], cnt[i])) % mod;

    cout << res << "\n";

    return 0;
}