Word Search II

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.

public class Solution {
    public List<String> findWords(char[][] board, String[] words) {
        if (board == null || board.length == 0 || board[0].length == 0 || words == null || words.length == 0) {
   //         return new ArrayList<String>();
        }
        Trie trie = new Trie();
        for (String word : words) {
            trie.insert(word);
        }
        Set<String> res = new HashSet<String>();
        int row = board.length;
        int column = board[0].length;
        boolean visit[][] = new boolean[row][column];
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < column; j++) {
                dfs(board, visit, "", i, j, trie, res);
            }
        }
        return new ArrayList<String>(res);
    }
   
    private void dfs(char[][] board, boolean[][] visit, String str, int i, int j, Trie trie, Set<String> set) {
        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length || visit[i][j]) {
            return;
        }
        str += board[i][j];
       
        if (!trie.startsWith(str)) {
            return;
        }
       
        if (trie.search(str)) {
            set.add(str);
        }

        visit[i][j] = true;
        dfs(board, visit, str, i + 1, j, trie, set);
        dfs(board, visit, str, i - 1, j, trie, set);
        dfs(board, visit, str, i, j + 1, trie, set);
        dfs(board, visit, str, i, j - 1, trie, set);
        visit[i][j] = false;
    }
}

class TrieNode {
    public TrieNode[] children = new TrieNode[26];
    public String item = "";

    // Initialize your data structure here.
    public TrieNode() {
    }
}

class Trie {
    private TrieNode root;

    public Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            if (node.children[c - 'a'] == null) {
                node.children[c - 'a'] = new TrieNode();
            }
            node = node.children[c - 'a'];
        }
        node.item = word;
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            if (node.children[c - 'a'] == null) return false;
            node = node.children[c - 'a'];
        }
        return node.item.equals(word);
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for (char c : prefix.toCharArray()) {
            if (node.children[c - 'a'] == null) return false;
            node = node.children[c - 'a'];
        }
        return true;
    }
}