Shared code

/*
  author: @ankingcodes
  created: 2021-08-08 08:43:09.737451
*/
        
#include<bits/stdc++.h>
#include<algorithm>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace std;
#define ll long long
#define INF 1e18
#define f first
#define s second
#define mp make_pair
#define pb push_back
const int MOD = (int) 1e9 + 7;

typedef tree<int, null_type, less<int>, rb_tree_tag,
                tree_order_statistics_node_update> PBDS;

typedef tree<pair<int, int>, null_type, less<pair<int, int>>, rb_tree_tag,
                tree_order_statistics_node_update> pairPBDS;

int main(){
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m; cin >> n >> m;
  int dp[n+1][m+1] = {};
  // special case: first index 
  int prev;
  int x; cin >> x;
  if (x == 0) for (int i=0;i<=m;i++) dp[0][i] = 1;
  else dp[0][x] = 1;
  // for i > 0, no. of ways to fill up to i, is sum of prev val
  for (int i=1;i<n;i++) {
    int x; cin >> x;
    prev = x;
    if (x == 0) {
      for (int j=1;j<=m;j++) {
        for (int k: {j-1, j, j+1}) 
          if (k >= 1 && k <= m) 
            (dp[i][j] += dp[i-1][k])%=MOD;
      }
    } else {
      for (int k: {x-1, x, x+1})
        if (k >= 1 && k <= m)
          (dp[i][x] += dp[i-1][k])%=MOD;
    }
  }
  // why this step ?
  int ans = 0;
  for (int i=1;i<=m;i++)
    (ans += dp[n-1][i])%=MOD;
  cout << ans << endl;
  return 0;
}