Formatted question description: https://leetcode.ca/all/790.html

# 790. Domino and Tromino Tiling

Medium

## Description

We have two types of tiles: a 2x1 domino shape, and an “L” tromino shape. These shapes may be rotated.

XX <- domino

XX <- "L" tromino
X


Given N, how many ways are there to tile a 2 x N board? Return your answer modulo 10^9 + 7.

(In a tiling, every square must be covered by a tile. Two tilings are different if and only if there are two 4-directionally adjacent cells on the board such that exactly one of the tilings has both squares occupied by a tile.)

Example:

Input: 3
Output: 5
Explanation:
The five different ways are listed below, different letters indicates different tiles:
XYZ XXZ XYY XXY XYY
XYZ YYZ XZZ XYY XXY


Note:

• N will be in range [1, 1000].

## Solution

Use dynamic programming. For N <= 2, the results are straightforward. If N equals 0 or 1, the result is 1 (note that when N equals 0, no tile is needed, which counts for one way). If N equals 2, the result is 2.

For N >= 3, the board can be tiled by adding one vertical domino after N - 1 columns, which counts for one way, or adding two horizontal dominoes after N - 2 columns, which counts for one way, or adding two trominoes and several dominoes after N - k columns where 3 <= k <= N, which count for two ways for each k.

Let f(N) be the number of ways to tile a 2 x N board. It can be seen that f(0) = 1, f(1) = 1, f(2) = 2, and for N >= 3, f(N) = f(N - 1) + f(N - 2) + (f(0) + f(1) + … + f(N - 3)) * 2. Apply the same recurrence formula for f(N - 1), and it can be induced that for N >= 3, f(N) = f(N - 1) * 2 + f(N - 3). Apply this formula to obtain f(N).

• class Solution {
public int numTilings(int N) {
if (N == 0)
return 1;
if (N <= 2)
return N;
final int MODULO = 1000000007;
int prev2 = 1, prev1 = 1, cur = 2;
for (int i = 3; i <= N; i++) {
int next = (cur * 2 % MODULO + prev2) % MODULO;
prev2 = prev1;
prev1 = cur;
cur = next;
}
return cur;
}
}

• // OJ: https://leetcode.com/problems/domino-and-tromino-tiling/
// Time: O(N)
// Space: O(N^2)
class Solution {
public:
int numTilings(int n) {
long mod = 1e9 + 7;
vector<vector<long>> dp(n + 2, vector<long>(n + 2));
dp[1][1] = 1;
for (int i = 2; i <= n + 1; ++i) {
dp[i][i] = (dp[i - 1][i - 1] + dp[i - 1][i - 2] * 2 + dp[i - 2][i - 2]) % mod;
dp[i][i - 1] = (dp[i - 1][i - 2] + dp[i - 2][i - 2]) % mod;
}
return dp[n + 1][n + 1];
}
};

• class Solution:
def numTilings(self, N):
"""
:type N: int
:rtype: int
"""
dp = [[0] * 2 for _ in range(N + 1)]
dp[0][0] = 1
dp[1][0] = 1
for i in range(2, N + 1):
dp[i][0] = (dp[i - 1][0] + dp[i - 2][0] + 2 * dp[i - 1][1]) % (10 ** 9 + 7)
dp[i][1] = (dp[i - 2][0] + dp[i - 1][1]) % (10 ** 9 + 7)
return dp[-1][0]