leetcode212

 class Solution {
     public List<String> findWords(char[][] board, String[] words) {
     List<String> res = new ArrayList<>();
     TrieNode root = buildTrie(words);
     for (int i = 0; i < board.length; i++) {
         for (int j = 0; j < board[0].length; j++) {
             dfs (board, i, j, root, res);
         }
     }
     return res;
 }

 public void dfs(char[][] board, int i, int j, TrieNode p, List<String> res) {
     char c = board[i][j];
     if (c == '#' || p.next[c - 'a'] == null) return;
     p = p.next[c - 'a'];
     if (p.word != null) {   // found one
         res.add(p.word);
         p.word = null;     // de-duplicate
     }

     board[i][j] = '#';
     if (i > 0) dfs(board, i - 1, j ,p, res);
     if (j > 0) dfs(board, i, j - 1, p, res);
     if (i < board.length - 1) dfs(board, i + 1, j, p, res);
     if (j < board[0].length - 1) dfs(board, i, j + 1, p, res);
     board[i][j] = c;
 }

 public TrieNode buildTrie(String[] words) {
     TrieNode root = new TrieNode();
     for (String w : words) {
         TrieNode p = root;
         for (char c : w.toCharArray()) {
             int i = c - 'a';
             if (p.next[i] == null) p.next[i] = new TrieNode();
             p = p.next[i];
        }
        p.word = w;
     }
     return root;
 }

 class TrieNode {
     TrieNode[] next = new TrieNode[26];
     String word;
 }
 }

参考：https://leetcode.com/problems/word-search-ii/discuss/59780/Java-15ms-Easiest-Solution-(100.00)