LWC 73: 790. Domino and Tromino Tiling
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LWC 73: 790. Domino and Tromino Tiling
传送门:790. Domino and Tromino Tiling
Problem:
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].
思路:
动态规划,切分子问题。比如考虑N等于1可能出现的状态:
从上至下对应状态为0, 1, 2, 3,因此可以得到N=2时,每个状态的转移方程:
dp[i][0] = dp[i - 1][0] + dp[i - 1][3] + dp[i - 2][1] + dp[i - 2][2]
dp[i][1] = dp[i - 1][0] + dp[i - 1][2]
dp[i][2] = dp[i - 1][0] + dp[i - 1][1]
dp[i][3] = dp[i - 1][0]
代码如下:
public int numTilings(int N)
long[][] dp = new long[N+1][4];
int mod = 1000000007;
dp[1][0] = 1;
dp[1][1] = 1;
dp[1][2] = 1;
dp[1][3] = 1;
for (int i = 2; i <= N; ++i)
dp[i][0] = (dp[i - 1][0] + dp[i - 1][3] + dp[i - 2][1] + dp[i - 2][2]) % mod;
dp[i][1] = (dp[i - 1][0] + dp[i - 1][2]) % mod;
dp[i][2] = (dp[i - 1][0] + dp[i - 1][1]) % mod;
dp[i][3] = dp[i - 1][0] % mod;
return (int)dp[N][0];
Python版本:
class Solution(object):
def numTilings(self, N):
"""
:type N: int
:rtype: int
"""
dp = [[0] * 4 for _ in range(N + 1)]
mod = 1000000007
dp[1][0] = dp[1][1] = dp[1][2] = dp[1][3] = 1
for i in range(2, N + 1):
dp[i][0] = (dp[i - 1][0] + dp[i - 1][3] + dp[i - 2][1] + dp[i - 2][2]) % mod
dp[i][1] = (dp[i - 1][0] + dp[i - 1][2]) % mod
dp[i][2] = (dp[i - 1][0] + dp[i - 1][1]) % mod
dp[i][3] = dp[i - 1][0] % mod
return dp[N][0]
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