LeetCode: Word Search II

LeetCode: 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 =

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[
  ['o','a','a','n'],
  ['e','t','a','e'],
  ['i','h','k','r'],
  ['i','f','l','v']
]

Return ["eat","oath"].

To finish this, please implement this first: Implement Trie (Prefix Tree)

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public class WordSearchII {
    public List<String> findWords(char[][] board, String[] words) {
        if(board == null || board.length == 0 || words == null || words.length == 0) return Collections.emptyList();
        Trie trie = new Trie();
        for(String s: words) trie.insert(s);

        Set<String> result = new HashSet<>();
        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++){
                helper(result, board, i, j, trie, visited, "");
            }
        }
        return new ArrayList<>(result);
    }

    private void helper(Set<String> result, char[][] board, int x, int y, Trie trie, boolean[][] visited, String out){
        if(x <0 || y<0 || x>= board.length || y>=board[0].length || visited[x][y]) return;
        out += board[x][y];
        if(!trie.startsWith(out)) return;
        if(trie.search(out)) result.add(out);

        visited[x][y] = true;
        helper(result, board, x+1, y, trie, visited, out);
        helper(result, board, x-1, y, trie, visited, out);
        helper(result, board, x, y+1, trie, visited, out);
        helper(result, board, x, y-1, trie, visited, out);
        visited[x][y] = false;
    }

    private static class Trie {

        private TrieNode root = new TrieNode();

        void insert(String s){
            if(s == null || s.isEmpty()) return;

            Map<Character, TrieNode>  children = root.children;
            for(int i = 0;i<s.length();i++){
                char c = s.charAt(i);
                TrieNode node = children.getOrDefault(c, new TrieNode());
                node.val = c;
                if(i + 1 == s.length()) node.isLeaf = true;
                children.put(c, node);
                children = node.children;
            }
        }

        boolean search(String s){
            if(s == null || s.isEmpty()) return true;
            Map<Character, TrieNode>  children = root.children;
            TrieNode node = null;
            for(char c : s.toCharArray()){
                if(!children.containsKey(c)) return false;
                node = children.get(c);
                children = node.children;
            }
            return node!=null && node.isLeaf;
        }

        boolean startsWith(String s){
            if(s == null || s.isEmpty()) return true;
            Map<Character, TrieNode>  children = root.children;
            for(char c : s.toCharArray()){
                if(!children.containsKey(c)) return false;
                children = children.get(c).children;
            }
            return true;
        }
    }

    private static class TrieNode {
        char val;
        Map<Character, TrieNode> children = new HashMap<>();
        boolean isLeaf = false;
    }
}