Given n orders, each order consist in pickup and delivery services. 
Count all valid pickup/delivery possible sequences such that delivery(i) is always after of pickup(i). 
Since the answer may be too large, return it modulo 10^9 + 7.
 
Example 1:
Input: n = 1
Output: 1
Explanation: Unique order (P1, D1), Delivery 1 always is after of Pickup 1.

Example 2:
Input: n = 2
Output: 6
Explanation: All possible orders: 
(P1,P2,D1,D2), (P1,P2,D2,D1), (P1,D1,P2,D2), (P2,P1,D1,D2), (P2,P1,D2,D1) and (P2,D2,P1,D1).
This is an invalid order (P1,D2,P2,D1) because Pickup 2 is after of Delivery 2.

Example 3:
Input: n = 3
Output: 90

 
Constraints:

1 <= n <= 500

Difficulty: MEDIUM
Tags: Dynamic programming

Combinatorics

Mathematics
Skill types: Dynamic programming

Starter code:
class Solution:
    def countOrders(self, n: int) -> int:
        

Solutions:
class Solution:

	def countOrders(self, n: int) -> int:
		if n == 1:
			return 1
		p = (n - 1) * 2 + 1
		dp = [0 for i in range(n + 1)]
		dp[1] = 1
		M = 10 ** 9 + 7
		for i in range(2, n + 1):
			p = (i - 1) * 2 + 1
			dp[i] = dp[i - 1] % M * (p * (p + 1) // 2) % M % M
		return dp[n]


class Solution:

	def countOrders(self, n: int) -> int:
		ans = 1
		MOD = 1000000007
		for i in range(2, n + 1):
			ans *= (2 * i - 1) * (i - 1) + (2 * i - 1)
			ans %= MOD
		return ans


class Solution:

	def countOrders(self, n: int) -> int:
		if n <= 1:
			return n
		num_spots = n * 2 - 1
		return num_spots * (num_spots + 1) // 2 * self.countOrders(n - 1) % (10 ** 9 + 7)


class Solution:

	def countOrders(self, n: int) -> int:
		F = [0, 1]
		for i in range(2, n + 1):
			one_way = i * 2 - 1
			F.append(one_way * F[i - 1] * i)
		return F[n] % (pow(10, 9) + 7)


import math

class Solution:

	def countOrders(self, n: int) -> int:
		MOD = 10 ** 9 + 7
		dp = [0 for _ in range(n + 1)]
		dp[1] = 1
		for i in range(2, n + 1):
			N = (i - 1) * 2 + 1
			dp[i] = (N + math.factorial(N) // (math.factorial(N - 2) * 2)) * dp[i - 1] % MOD
		return dp[n]


class Solution:

	def countOrders(self, n: int) -> int:
		dp = {}
		dp[1] = 1
		for i in range(2, n + 1):
			(num_digit, res) = (2 * (i - 1), 0)
			for j in range(num_digit + 1, 0, -1):
				res += j
			dp[i] = res * dp[i - 1]
		return dp[n] % (10 ** 9 + 7)


class Solution:

	def countOrders(self, n: int) -> int:
		dp = [0 for _ in range(n + 1)]
		dp[1] = 1
		mod = 10 ** 9 + 7
		for i in range(2, n + 1):
			spaces = (i - 1) * 2 + 1
			permutations = sum((j for j in range(1, spaces + 1)))
			dp[i] = dp[i - 1] * permutations % mod
		return dp[n]


class Solution:

	def countOrders(self, n: int) -> int:
		if n == 1:
			return 1
		else:
			positions = 2 * (n - 1) + 1
			all_possib = 0
			for i in range(positions + 1):
				possibilities = positions - i
				all_possib += possibilities
			return all_possib * self.countOrders(n - 1) % 1000000007


class Solution:

	def countOrders(self, n: int) -> int:
		dp = [1 for i in range(n)]
		for i in range(1, n):
			sequence = i + 1
			length = 2 * i
			temp = sum([j for j in range(1, length + 2)])
			dp[i] = temp * dp[i - 1]
		print(dp)
		return dp[-1] % (10 ** 9 + 7)


class Solution:

	def countOrders(self, n: int) -> int:
		mod = 10 ** 9 + 7
		facts = [1]
		for i in range(1, n + 1):
			facts.append(facts[-1] * i)
		dp = [[0] * (n + 1) for _ in range(n + 1)]
		for p in range(n + 1):
			for d in range(n - p + 1):
				if p > 0 and d > 0:
					dp[p][d] += dp[p - 1][d + 1] * p % mod
					dp[p][d] += dp[p][d - 1] * d % mod
				elif p > 0 and d == 0:
					dp[p][d] += dp[p - 1][d + 1] * p % mod
				elif d > 0 and p == 0:
					dp[p][d] += facts[d] % mod
		return dp[n][0]


class Solution:

	def countOrders(self, n: int) -> int:
		MOD = int(1000000000.0) + 7
		d = [[0] * (n + 2) for _ in range(n + 2)]
		d[0][0] = 1
		for i in range(n + 1):
			for j in range(i + 1):
				d[i + 1][j] = (d[i + 1][j] + d[i][j] * (n - i)) % MOD
				d[i][j + 1] = (d[i][j + 1] + d[i][j] * (i - j)) % MOD
		return d[n][n - 1]


class Solution:

	def countOrders(self, n: int) -> int:
		mod = 10 ** 9 + 7
		facts = [1]
		for i in range(1, n + 1):
			facts.append(facts[-1] * i)

		@lru_cache(None)
		def ways(pickOptions, deliverOptions):
			nonlocal mod
			ans = 0
			if pickOptions > 0 and deliverOptions > 0:
				ans += ways(pickOptions - 1, deliverOptions + 1) * pickOptions
				ans += ways(pickOptions, deliverOptions - 1) * deliverOptions
			elif pickOptions > 0 and deliverOptions == 0:
				ans += ways(pickOptions - 1, deliverOptions + 1) * pickOptions
			elif deliverOptions > 0 and pickOptions == 0:
				ans += facts[deliverOptions]
			return ans % mod
		return ways(n, 0)


class Solution:

	def countOrders(self, n: int) -> int:
		l = 2
		num = 1
		for i in range(2, n + 1):
			num = sum((num * (l - hole + 1) for hole in range(l + 1)))
			l += 2
		return num % (10 ** 9 + 7)


class Solution:

	def countOrders(self, n: int) -> int:

		@lru_cache(None)
		def factorial(x):
			fact = 1
			for i in range(1, x + 1):
				fact *= i
			return fact
		mod = 10 ** 9 + 7

		@lru_cache(None)
		def ways(pickOptions, deliverOptions):
			nonlocal mod
			ans = 0
			if pickOptions > 0 and deliverOptions > 0:
				ans += ways(pickOptions - 1, deliverOptions + 1) * pickOptions
				ans += ways(pickOptions, deliverOptions - 1) * deliverOptions
			elif pickOptions > 0 and deliverOptions == 0:
				ans += ways(pickOptions - 1, deliverOptions + 1) * pickOptions
			elif deliverOptions > 0 and pickOptions == 0:
				ans += factorial(deliverOptions)
			return ans % mod
		return ways(n, 0)


class Solution:

	def countOrders(self, n: int) -> int:
		pickup = n
		delivery = 0

		def factorial(x):
			fact = 1
			for i in range(1, x + 1):
				fact *= i
			return fact
		mod = 10 ** 9 + 7

		@lru_cache(None)
		def ways(pick, deliver):
			nonlocal mod
			ans = 0
			if pick > 0 and deliver > 0:
				ans += ways(pick - 1, deliver + 1) * pick
				ans += ways(pick, deliver - 1) * deliver
			elif pick > 0 and deliver == 0:
				ans += ways(pick - 1, deliver + 1) * pick
			elif deliver > 0 and pick == 0:
				ans += factorial(deliver)
			return ans % mod
		return ways(n, 0)


class Solution:

	def countOrders(self, n: int) -> int:
		DP = [0] * (n + 1)
		DP[1] = 1
		for i in range(2, n + 1):
			newpos = (i * 2 - 1 + 1) * (i * 2 - 1) // 2
			DP[i] = newpos * DP[i - 1]
		return DP[-1] % (10 ** 9 + 7)


class Solution:

	def countOrders(self, n: int) -> int:

		def solve(remaining, pending):
			if remaining == 0 and pending == 1:
				return 1
			if memo[remaining][pending] != -1:
				return memo[remaining][pending]
			count = 0
			if remaining > 0:
				count += remaining * solve(remaining - 1, pending + 1)
			if pending > 0:
				count += pending * solve(remaining, pending - 1)
			count %= MOD
			memo[remaining][pending] = count
			return count
		MOD = 10 ** 9 + 7
		memo = [[-1 for _ in range(n + 1)] for _ in range(n + 1)]
		return solve(n, 0)


class Solution:

	def countOrders(self, n: int) -> int:
		opt = [[0 for _ in range(n + 1)] for _ in range(n + 1)]
		for j in range(n + 1):
			for i in range(n + 1):
				if j == n:
					opt[i][j] = j * opt[i + 1][j - 1]
					return opt[0][n] % (pow(10, 9) + 7)
				if i == 0 and j == 0:
					continue
				if i == 0 and j == 1:
					opt[i][j] = 1
					continue
				if i == 1 and j == 0:
					opt[i][j] = 1
					continue
				if i == 0:
					opt[i][j] = j * opt[i + 1][j - 1]
				elif j == 0:
					opt[i][j] = i * opt[i - 1][j]
				else:
					if i + j > n:
						break
					opt[i][j] = i * opt[i - 1][j] + j * opt[i + 1][j - 1]


class Solution:

	def countOrders(self, n: int) -> int:
		orderResults = {}

		def orderHelper(pickupsLeft, heldPackages):
			key = (pickupsLeft, heldPackages)
			if key in orderResults:
				return orderResults[key]
			if key == (0, 0):
				return 0
			elif key == (0, 1):
				return 1
			elif key == (1, 0):
				return 1
			res = 0
			if pickupsLeft > 0:
				res += pickupsLeft * orderHelper(pickupsLeft - 1, heldPackages + 1)
			if heldPackages > 0:
				res += heldPackages * orderHelper(pickupsLeft, heldPackages - 1)
			res = res % (10 ** 9 + 7)
			orderResults[key] = res
			return res
		return orderHelper(n, 0)


from itertools import permutations

class Solution:

	def countOrders(self, n: int) -> int:
		dp = {}
		MOD = 10 ** 9 + 7
		dp[0, 0] = 1

		def helper(pick, deliver):
			nonlocal dp
			key = (pick, deliver)
			if key in dp:
				return dp[key]
			elif pick > deliver:
				return 0
			ans = 0
			if pick > 0:
				ans += helper(pick - 1, deliver) * pick % MOD
			if deliver > 0 and pick < deliver:
				ans += helper(pick, deliver - 1) * (deliver - pick) % MOD
			dp[key] = ans
			return ans
		return helper(n, n) % MOD


def oneMore(length: int) -> int:
	if length < 2:
		return 1
	return int(length * (length + 1) / 2 + length + 1)

class Solution:

	def countOrders(self, n: int) -> int:
		acc = 1
		for i in range(0, n * 2, 2):
			acc *= oneMore(i)
			acc %= int(1000000000.0 + 7)
		return acc


class Solution:
	import sys

	def countOrders(self, n: int) -> int:
		print(sys.getrecursionlimit())

		@lru_cache(None)
		def helper(p, d):
			if p == n and d == n:
				return 1
			ans = 0
			if n > p:
				ans += (n - p) * helper(p + 1, d)
			if p > d:
				ans += (p - d) * helper(p, d + 1)
			return ans
		return helper(0, 0) % (10 ** 9 + 7)


class Solution:

	def countOrders(self, n: int) -> int:
		res = 1
		for i in range(1, n + 1):
			res = res * self.choose(2 * i, 2)
		return res % int(math.pow(10, 9) + 7)

	def choose(self, n, k):
		if 0 <= k <= n:
			ntok = 1
			ktok = 1
			for t in range(1, min(k, n - k) + 1):
				ntok *= n
				ktok *= t
				n -= 1
			return ntok // ktok
		else:
			return 0


class Solution:

	def countOrders(self, n: int) -> int:
		mod = 10 ** 9 + 7
		res = 1
		for i in range(2 * n, 0, -1):
			res = res * i
			if i & 1:
				res /= 2
				res = res % mod
		return int(res)


class Solution:

	def countOrders(self, n: int) -> int:
		MOD = 10 ** 9 + 7
		res = 1
		for i in range(2, n + 1):
			res = res * (2 * i - 1) * (2 * i) // 2 % MOD
		return res


class Solution:

	def countOrders(self, n: int) -> int:
		res = 1
		for i in range(1, n + 1):
			x = (i - 1) * 2 + 1
			possibilities = x * (x - 1) // 2 + x
			res = res * possibilities % (1000000000 + 7)
		return res