212. Word Search II

就是dfs+trie

1.先建立一个trie，把字典里所有的词加到trie里

2.对于板子里面的每个格子开始，向四个方向搜索，每次到了一个新的格子，添加在之前的单词上，然后检查trie，如果没有以这个开头的词，那就返回，如果包含了这个词，就加到结果里

要注意的是，即使包含这个词，还是要继续往下走，比如“aa”和“aab”都在词典中，不可以找到“aa”就停止了

 

public class Solution {
    static final int[][] dirs = {{-1, 0}, {1, 0}, {0, 1}, {0, -1}};
    
    public List<String> findWords(char[][] board, String[] words) {
        Set<String> res = new HashSet<String>();
        Trie trie = new Trie();
        boolean[][] visited = new boolean[board.length][board[0].length];
        for(int i = 0; i < words.length; i++) {
            trie.insert(words[i]);
        }
        for(int i = 0; i < board.length; i++) {
            for(int j = 0; j < board[0].length; j++) {
                dfs(res, board, "", i, j, trie, visited);
            }
        }
        return new ArrayList<String>(res);
    }
    
    private void dfs(Set<String> res, char[][] board, String word, int x, int y, Trie trie, boolean[][] visited) {
        if(x < 0 || y < 0 || x >= board.length || y >= board[0].length || visited[x][y]) {
            return;
        }
        word += board[x][y];
        if(!trie.startsWith(word)) {
            return;
        }
        if(trie.search(word)) {
            res.add(word);
        }
        visited[x][y] = true;
        for(int[] dir: dirs) {
            dfs(res, board, word, x + dir[0], y + dir[1], trie, visited);
        }
        visited[x][y] = false;
    }
    
    class TrieNode {
        char c;
        boolean isLeaf;
        TrieNode[] next;
        // Initialize your data structure here.
        public TrieNode() {
            next = new TrieNode[26];
            isLeaf = false;
        }
    }
    
    public class Trie {
        private TrieNode root;
    
        public Trie() {
            root = new TrieNode();
            
        }
    
        // Inserts a word into the trie.
        public void insert(String word) {
            if(word == null || word.length() == 0) {
                return;
            }
            TrieNode cur = root;
            for(int i = 0; i < word.length(); i++) {
                char c = word.charAt(i);
                int index = c - 'a';
                if(cur.next[index] == null) {
                    cur.next[index] = new TrieNode();
                    cur.next[index].c = c;
                }
                cur = cur.next[index];
            }
            cur.isLeaf = true;
        }
    
        // Returns if the word is in the trie.
        public boolean search(String word) {
            if(word == null || word.length() == 0) {
                return true;
            }
            TrieNode cur = root;
            for(int i = 0; i < word.length(); i++) {
                char c = word.charAt(i);
                int index = c - 'a';
                if(cur.next[index] == null) {
                    return false;
                }
                cur = cur.next[index];
            }
            return cur.isLeaf;
        }
    
        // 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 true;
            }
            TrieNode cur = root;
            for(int i = 0; i < prefix.length(); i++) {
                char c = prefix.charAt(i);
                int index = c - 'a';
                if(cur.next[index] == null) {
                    return false;
                }
                cur = cur.next[index];
            }
            return true;
        }
    }
            
}