题目：
Given a 2D board and a list of words from the dictionary, find all words in the board.
Each word must be constructed from letters of sequentially adjacent cell, where "adjacent" cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.
For example,
Given words = ["oath","pea","eat","rain"] and board =
[ [‘o‘,‘a‘,‘a‘,‘n‘], [‘e‘,‘t‘,‘a‘,‘e‘], [‘i‘,‘h‘,‘k‘,‘r‘], [‘i‘,‘f‘,‘l‘,‘v‘] ]
Return ["eat","oath"].
Note:
You may assume that all inputs are consist of lowercase letters a-z.
You would need to optimize your backtracking to pass the larger test. Could you stop backtracking earlier?
If the current candidate does not exist in all words‘ prefix, you could stop backtracking immediately. What kind of data structure could answer such query efficiently? Does a hash table work? Why or why not? How about a Trie? If you would like to learn how to implement a basic trie, please work on this problem: Implement Trie (Prefix Tree) first.
链接： http://leetcode.com/problems/word-search-ii/
题解：
使用Word Search的方法会超时。因为对每一个word我们都要对整个board进行一遍dfs，所以对于每个单词我们的Time Complexity - O(mn * 26L)，大集合的话时间会很长，所以不能用这个方法。于是根据提示我们尝试使用Tire来做。假如用所有的word先建立Trie，然后再用DFS扫描board就可以了。
Time Complexity - O(mn * L)， Space Complexity - O(26L)
Reference:
https://leetcode.com/discuss/36411/27-lines-uses-complex-numbers
原文：http://www.cnblogs.com/yrbbest/p/4979650.html