leetcode 756矩阵转换金字塔

题目：
Share
We are stacking blocks to form a pyramid. Each block has a color which is a one letter string, like 'Z'.

For every block of color C we place not in the bottom row, we are placing it on top of a left block of color A and right block of color B. We are allowed to place the block there only if (A, B, C) is an allowed triple.

We start with a bottom row of bottom, represented as a single string. We also start with a list of allowed triples allowed. Each allowed triple is represented as a string of length 3.

Return true if we can build the pyramid all the way to the top, otherwise false.

Example 1:
Input: bottom = “XYZ”, allowed = [“XYD”, “YZE”, “DEA”, “FFF”]
Output: true
Explanation:
We can stack the pyramid like this:
A
/
D E
/ \ /
X Y Z

This works because (‘X’, ‘Y’, ‘D’), (‘Y’, ‘Z’, ‘E’), and (‘D’, ‘E’, ‘A’) are allowed triples.
Example 2:
Input: bottom = “XXYX”, allowed = [“XXX”, “XXY”, “XYX”, “XYY”, “YXZ”]
Output: false
Explanation:
We can’t stack the pyramid to the top.
Note that there could be allowed triples (A, B, C) and (A, B, D) with C != D.
Note:
bottom will be a string with length in range [2, 8].
allowed will have length in range [0, 200].
Letters in all strings will be chosen from the set {‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’}.
Accepted
10,413
Submissions
20,933
题解：
def pyramidTransition(self, bottom, allowed):
“”"
:type bottom: str
:type allowed: List[str]
:rtype: bool
“”"
from itertools import product
from collections import defaultdict
f=defaultdict(lambda:defaultdict(list))
for u,v,w in allowed:
f[u][v].append(w)
def pyramid(bottom):
return len(bottom)==1 or any(pyramid(i) for i in product(*(f[a][b] for a,b in zip(bottom,bottom[1:]))))
return pyramid(bottom)