最快的解决方案可能是将字典存储在一个trie中。然后，创建一个三元组队列(x, y, s)，其中队列中的每个元素对应于一个可以在网格中拼写的单词的前缀s，结束于位置(x, y)。初始化队列中有N x N个元素(其中N是网格的大小)，网格中的每个正方形都有一个元素。然后，算法进行如下:
While the queue is not empty:
Dequeue a triple (x, y, s)
For each square (x', y') with letter c adjacent to (x, y):
If s+c is a word, output s+c
If s+c is a prefix of a word, insert (x', y', s+c) into the queue
如果将字典存储在trie中，则可以在常数时间内测试s+c是否是单词或单词的前缀(前提是还在每个队列数据中保留一些额外的元数据，例如指向trie中当前节点的指针)，因此此算法的运行时间为O(可拼写的单词数量)。
[编辑]下面是我刚刚编写的Python实现:
#!/usr/bin/python
class TrieNode:
def __init__(self, parent, value):
self.parent = parent
self.children = [None] * 26
self.isWord = False
if parent is not None:
parent.children[ord(value) - 97] = self
def MakeTrie(dictfile):
dict = open(dictfile)
root = TrieNode(None, '')
for word in dict:
curNode = root
for letter in word.lower():
if 97 <= ord(letter) < 123:
nextNode = curNode.children[ord(letter) - 97]
if nextNode is None:
nextNode = TrieNode(curNode, letter)
curNode = nextNode
curNode.isWord = True
return root
def BoggleWords(grid, dict):
rows = len(grid)
cols = len(grid[0])
queue = []
words = []
for y in range(cols):
for x in range(rows):
c = grid[y][x]
node = dict.children[ord(c) - 97]
if node is not None:
queue.append((x, y, c, node))
while queue:
x, y, s, node = queue[0]
del queue[0]
for dx, dy in ((1, 0), (1, -1), (0, -1), (-1, -1), (-1, 0), (-1, 1), (0, 1), (1, 1)):
x2, y2 = x + dx, y + dy
if 0 <= x2 < cols and 0 <= y2 < rows:
s2 = s + grid[y2][x2]
node2 = node.children[ord(grid[y2][x2]) - 97]
if node2 is not None:
if node2.isWord:
words.append(s2)
queue.append((x2, y2, s2, node2))
return words
使用示例:
d = MakeTrie('/usr/share/dict/words')
print(BoggleWords(['fxie','amlo','ewbx','astu'], d))
输出:
['fa', 'xi', 'ie', 'io', 'el', 'am', 'ax', 'ae', 'aw', 'mi', 'ma', 'me', 'lo', 'li', 'oe', 'ox', 'em', 'ea', 'ea', 'es', 'wa', 'we', 'wa', 'bo', 'bu', 'as', 'aw', 'ae', 'st', 'se', 'sa', 'tu', 'ut', 'fam', 'fae', 'imi', 'eli', 'elm', 'elb', 'ami', 'ama', 'ame', 'aes', 'awl', 'awa', 'awe', 'awa', 'mix', 'mim', 'mil', 'mam', 'max', 'mae', 'maw', 'mew', 'mem', 'mes', 'lob', 'lox', 'lei', 'leo', 'lie', 'lim', 'oil', 'olm', 'ewe', 'eme', 'wax', 'waf', 'wae', 'waw', 'wem', 'wea', 'wea', 'was', 'waw', 'wae', 'bob', 'blo', 'bub', 'but', 'ast', 'ase', 'asa', 'awl', 'awa', 'awe', 'awa', 'aes', 'swa', 'swa', 'sew', 'sea', 'sea', 'saw', 'tux', 'tub', 'tut', 'twa', 'twa', 'tst', 'utu', 'fama', 'fame', 'ixil', 'imam', 'amli', 'amil', 'ambo', 'axil', 'axle', 'mimi', 'mima', 'mime', 'milo', 'mile', 'mewl', 'mese', 'mesa', 'lolo', 'lobo', 'lima', 'lime', 'limb', 'lile', 'oime', 'oleo', 'olio', 'oboe', 'obol', 'emim', 'emil', 'east', 'ease', 'wame', 'wawa', 'wawa', 'weam', 'west', 'wese', 'wast', 'wase', 'wawa', 'wawa', 'boil', 'bolo', 'bole', 'bobo', 'blob', 'bleo', 'bubo', 'asem', 'stub', 'stut', 'swam', 'semi', 'seme', 'seam', 'seax', 'sasa', 'sawt', 'tutu', 'tuts', 'twae', 'twas', 'twae', 'ilima', 'amble', 'axile', 'awest', 'mamie', 'mambo', 'maxim', 'mease', 'mesem', 'limax', 'limes', 'limbo', 'limbu', 'obole', 'emesa', 'embox', 'awest', 'swami', 'famble', 'mimble', 'maxima', 'embolo', 'embole', 'wamble', 'semese', 'semble', 'sawbwa', 'sawbwa']
Notes: This program doesn't output 1-letter words, or filter by word length at all. That's easy to add but not really relevant to the problem. It also outputs some words multiple times if they can be spelled in multiple ways. If a given word can be spelled in many different ways (worst case: every letter in the grid is the same (e.g. 'A') and a word like 'aaaaaaaaaa' is in your dictionary), then the running time will get horribly exponential. Filtering out duplicates and sorting is trivial to due after the algorithm has finished.
