LeetCode in Net

212. Word Search II

Hard

Given an m x n board of characters and a list of strings words, return all words on the board.

Each word must be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.

Example 1:

Input: board = [[“o”,”a”,”a”,”n”],[“e”,”t”,”a”,”e”],[“i”,”h”,”k”,”r”],[“i”,”f”,”l”,”v”]], words = [“oath”,”pea”,”eat”,”rain”]

Output: [“eat”,”oath”]

Example 2:

Input: board = [[“a”,”b”],[“c”,”d”]], words = [“abcb”]

Output: []

Constraints:

• m == board.length
• n == board[i].length
• 1 <= m, n <= 12
• board[i][j] is a lowercase English letter.
• 1 <= words.length <= 3 * 104
• 1 <= words[i].length <= 10
• words[i] consists of lowercase English letters.
• All the strings of words are unique.

Solution

using System.Collections.Generic;

public class Solution {
    private Tree root;

    public IList<string> FindWords(char[][] board, string[] words) {
        if (board.Length < 1 || board[0].Length < 1) {
            return new List<string>();
        }
        root = new Tree();
        foreach (var word in words) {
            Tree.AddWord(root, word);
        }
        var collected = new List<string>();
        for (int i = 0; i < board.Length; ++i) {
            for (int j = 0; j < board[0].Length; ++j) {
                Dfs(board, i, j, root, collected);
            }
        }
        return collected;
    }

    private void Dfs(char[][] board, int i, int j, Tree cur, List<string> collected) {
        char c = board[i][j];
        if (c == '-') {
            return;
        }
        cur = cur.GetChild(c);
        if (cur == null) {
            return;
        }
        if (cur.end != null) {
            string s = cur.end;
            collected.Add(s);
            cur.end = null;
            if (cur.Len() == 0) {
                Tree.DeleteWord(root, s);
            }
        }
        board[i][j] = '-';
        if (i > 0) {
            Dfs(board, i - 1, j, cur, collected);
        }
        if (i + 1 < board.Length) {
            Dfs(board, i + 1, j, cur, collected);
        }
        if (j > 0) {
            Dfs(board, i, j - 1, cur, collected);
        }
        if (j + 1 < board[0].Length) {
            Dfs(board, i, j + 1, cur, collected);
        }
        board[i][j] = c;
    }

    // Internal Trie/Tree class
    private class Tree {
        private readonly Tree[] children = new Tree[26];
        public string? end;

        public Tree GetChild(char c) {
            return children[c - 'a'];
        }

        public int Len() {
            int count = 0;
            foreach (var child in children) {
                if (child != null) {
                    count++;
                }
            }
            return count;
        }

        public static void AddWord(Tree root, string word) {
            var node = root;
            foreach (var c in word) {
                int idx = c - 'a';
                if (node.children[idx] == null) {
                    node.children[idx] = new Tree();
                }
                node = node.children[idx];
            }
            node.end = word;
        }

        public static void DeleteWord(Tree root, string word) {
            DeleteWordHelper(root, word, 0);
        }

        private static bool DeleteWordHelper(Tree node, string word, int d) {
            if (node == null) {
                return false;
            }
            if (d == word.Length) {
                node.end = null;
            }
            else {
                int idx = word[d] - 'a';
                if (DeleteWordHelper(node.children[idx], word, d + 1)) {
                    node.children[idx] = null;
                }
            }
            if (node.end != null) {
                return false;
            }
            foreach (var child in node.children) {
                if (child != null) return false;
            }
            return true;
        }
    }
}

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