212. Word Search II

    /*
     *212. Word Search II 
     *2016-6-11 by Mingyang
     *这真的是我见过最好玩的题目了，在已经有了Trie的基础上，我们再继续往下看
     *直接利用这个工具，每次都传进去，这样后面就可以不断的使用这个工具
     *然后首先的思路就是把Trie填满需要的东西
     *然后就是对每一个位置进行dfs。记住要使用visit map来进行定位
     */
    public List<String> findWords(char[][] board, String[] words) {
        HashSet<String> list = new HashSet();
        Tries trie = new Tries();
        for (String word : words)
            trie.insert(word);
        boolean[][] visited = new boolean[board.length][board[0].length];
        for (int i = 0; i < board.length; i++)
            for (int j = 0; j < board[0].length; j++)
                dfss(list, board, visited, "", i, j, trie);
        return new ArrayList(list);
    }
    public void dfss(Set<String> list, char[][] board, boolean[][] visited,
            String str, int x, int y, Tries trie) {
        if (x < 0 || x >= board.length || y < 0 || y >= board[0].length)
            return;
        if (visited[x][y])
            return;
        str += board[x][y];
        if (!trie.startsWith(str))
            return;
        if (trie.search(str))
            list.add(str);
        visited[x][y] = true;
        dfss(list, board, visited, str, x - 1, y, trie);
        dfss(list, board, visited, str, x + 1, y, trie);
        dfss(list, board, visited, str, x, y - 1, trie);
        dfss(list, board, visited, str, x, y + 1, trie);
        visited[x][y] = false;
    }
}
class Tries {
    public TrieNode root;
    public Tries() {
        root = new TrieNode();
        root.isEnd = true;
    }
    public void insert(String word) {
        if (word == null || word.length() == 0)
            return;
        TrieNode node = root;
        char[] letters = word.toCharArray();
        for (int i = 0; i < word.length(); i++) {
            int pos = letters[i] - 'a';
            if (node.son[pos] == null) {
                node.son[pos] = new TrieNode();
                node.son[pos].val = letters[i];
            }
            node = node.son[pos];
        }
        node.isEnd = true;
    }
    // Returns if the word is in the trie.
    public boolean search(String word) {
        if (word == null || word.length() == 0) {
            return false;
        }
        TrieNode node = root;
        char[] letters = word.toCharArray();
        for (int i = 0; i < word.length(); i++) {
            int pos = letters[i] - 'a';
            if (node.son[pos] != null) {
                node = node.son[pos];
            } else {
                return false;
            }
        }
        return node.isEnd;
    }
    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        if (prefix == null || prefix.length() == 0) {
            return false;
        }
        TrieNode node = root;
        char[] letters = prefix.toCharArray();
        for (int i = 0; i < prefix.length(); i++) {
            int pos = letters[i] - 'a';
            if (node.son[pos] != null) {
                node = node.son[pos];
            } else {
                return false;
            }
        }
        return true;
    }
}
class TrieNode {
    // Initialize your data structure here.
    TrieNode[] son;// 所有的儿子节点
    boolean isEnd;// 是不是最后一个节点
    char val;// 节点的值
    TrieNode() {
        this.son = new TrieNode[26];
        this.isEnd = false;
    }
}