212. Word Search II

package LeetCode_212

/**
 * 212. Word Search II
 * https://leetcode.com/problems/word-search-ii/
 * 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"]

Constraints:
1. m == board.length
2. n == board[i].length
3. 1 <= m, n <= 12
4. board[i][j] is a lowercase English letter.
5. 1 <= words.length <= 3 * 104
6. 1 <= words[i].length <= 10
7. words[i] consists of lowercase English letters.
8. All the strings of words are unique.
 * */
class Solution {
    /*
    * solution: TrieTree + DFS, save all word into Trie, search the board by DFS;
    * Time: O(m*n*4^l), l is length of word, because each letter has 4 path to check
    * Space: O(m*n)
    * */

    var root: TrieNode? = null

    init {
        root = TrieNode()
    }

    fun findWords(board: Array<CharArray>, words: Array<String>): List<String> {
        createTree(words)
        val result = ArrayList<String>()
        val m = board.size
        val n = board[0].size
        val visited = Array(m){BooleanArray(n)}
        for (i in 0 until m) {
            for (j in 0 until n) {
                dfs(board, i, j, "", result, visited)
            }
        }
        return result
    }

    private fun dfs(board: Array<CharArray>, x: Int, y: Int, s:String, result: ArrayList<String>, visited:Array<BooleanArray>) {
        if (x < 0 || x >= board.size || y < 0 || y >= board[0].size || visited[x][y]) {
            return
        }
        var cur = s
        cur += board[x][y]
        if (!startWith(cur)) {
            return
        }
        if (search(cur)) {
            //avoid duplicate
            if (!result.contains(cur)) {
                result.add(cur)
            }
        }
        visited[x][y] = true
        //search 4 directions
        dfs(board, x + 1, y, cur, result, visited)
        dfs(board, x - 1, y, cur, result, visited)
        dfs(board, x, y + 1, cur, result, visited)
        dfs(board, x, y - 1, cur, result, visited)
        //set back for next level, let it can go through again
        visited[x][y] = false
    }

    private fun startWith(word: String): Boolean {
        val node = find(word)
        if (node == null) {
            return false
        }
        return true
    }

    private fun search(word: String): Boolean {
        val node = find(word)
        if (node == null) {
            return false
        }
        return node.word.equals(word)
    }

    private fun find(word: String): TrieNode? {
        var cur = root
        for (c in word) {
            val index = c - 'a'
            if (cur!!.children[index] == null) {
                return null
            }
            cur = cur!!.children[index]
        }
        return cur
    }

    //create tree by word
    private fun createTree(words: Array<String>) {
        for (word in words) {
            var current = root
            for (c in word) {
                val index = c - 'a'
                if (current!!.children[index] == null) {
                    current.children[index] = TrieNode()
                }
                current = current.children[index]!!
            }
            //make last node as leaf
            current!!.word = word
        }
    }

    class TrieNode {
        var children = arrayOfNulls<TrieNode>(26)
        var word: String? = ""
    }
}