Alien Dictionary
要点:topological sort,bfs
- 只有前后两个word之间构成联系,一个word里的c是没有关系的
- 只要得到两个word第一个不同char之间的partial order即可。topological sort就是把partial order变为total order
错误点:
- ''.join(res)*(set(res)==chars)的含义:string*int,如果int是0那么实际返回的是''
- defaultdict usage: umap[k] and umap[k]=0是不同的,前者不会强制set为0
- don’t be too smart to compact everything: 比如可以开始建graph和indegree来保证所有char in (list comprehension can extend to dict comprehension)
- zip新用法:相邻两两比较:构建pair: 注意zip不是等长的list,只返回有效部分。
- 在构建graph的时候indegree[v]+=1对v已经在集合里的情况会重复++
# There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of words from the dictionary, where words are sorted lexicographically by the rules of this new language. Derive the order of letters in this language.
# For example,
# Given the following words in dictionary,
# [
# "wrt",
# "wrf",
# "er",
# "ett",
# "rftt"
# ]
# The correct order is: "wertf".
# Note:
# You may assume all letters are in lowercase.
# If the order is invalid, return an empty string.
# There may be multiple valid order of letters, return any one of them is fine.
# Hide Company Tags Google Airbnb Facebook Twitter Snapchat Pocket Gems
# Hide Tags Graph Topological Sort
# Hide Similar Problems (M) Course Schedule II
class Solution(object):
def alienOrder(self, words):
"""
:type words: List[str]
:rtype: str
"""
chars = set([c for word in words for c in word]) # error: inner loop at the end
graph, indegree = {c:set() for c in chars}, {c:0 for c in chars}
for pair in zip(words, words[1:]): # note: zip if not even, ignore redundant
w1,w2 = pair[0],pair[1]
i = 0
while i<len(w1) and i<len(w2):
if w1[i]!=w2[i]:
graph[w1[i]].add(w2[i])
# indegree[w2[i]]+=1 # error case: w2[i] already in set, wrongly count twice
break
i+=1
for u in graph:
for v in graph[u]:
indegree[v]+=1
#print graph, indegree
q = [c for c in indegree if indegree[c]==0]
res = []
while q:
next = []
for c in q:
res.append(c)
for n in graph[c]:
indegree[n]-=1
if indegree[n]==0:
next.append(n)
q = next
return ''.join(res)*(set(res)==chars) # error: don't forget cycle
sol = Solution()
assert sol.alienOrder(["ab","adc"])=="acbd"
assert sol.alienOrder(["wrt","wrf","er","ett","rftt"])=="wertf"
assert sol.alienOrder(["za","zb","ca","cb"])=="azbc"
assert sol.alienOrder(["ri","xz","qxf","jhsguaw","dztqrbwbm","dhdqfb","jdv","fcgfsilnb","ooby"])==""