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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
$n = int();
$a = ints();
$B = 0;
for($i = 0; $i < $n; $i++){
  $map[$i] = 0;
}
foreach($a as $b){
  $map[$b]++;
  $B = max($b, $B);
}
//echo $B;
//print_r($map);
$num = 1;
for($i = 1; $i <= $B; $i++){
  for($j = 0; $j < $map[$i]; $j++){
    $num *= $map[$i-1];
    $num %= 998244353;
  }
}
if($map[0] != 1)$num = 0;
if($a[0] != 0)$num = 0;
printf("%d", $num);

function str(){
  return trim(fgets(STDIN));
}
function ints(){
  return array_map("intval", explode(" ", trim(fgets(STDIN))));
}
function int(){
  return intval(trim(fgets(STDIN)));
}
function o(...$val){
    if(count($val)==1)$val = array_shift($val);
    $trace = debug_backtrace();
    echo $trace[0]['line'].")";
    if(is_array($val)){
        if(count($val) == 0){
            echo "empty array";
        }elseif(!is_array(current($val))){
            echo "array:";
            echo implode(" ", $val)."\n";
        }else{
            echo "array:array\n";
            foreach($val as $row){
                echo implode(" ", $row)."\n";
            }
        }
    }else{
        echo $val."\n";
    }
}