Add Find All Words On A Board

Author: marttp

kotlin/Tries/FindAllWordsOnABoard.kt

@@ -0,0 +1,64 @@
+/*
+    Definition of a TrieNode:
+
+    data class TrieNode(
+        var isWord: Boolean = false,
+        var word: String? = null
+    )
+*/
+
+fun findAllWordsOnABoard(board: Array<CharArray>, words: List<String>): List<String> {
+    val root = TrieNode()
+    // Insert every word into the trie.
+    for (word in words) {
+        var node = root
+        for (c in word.toCharArray()) {
+            if (c !in node.children) {
+                node.children[c] = TrieNode()
+            }
+            node = node.children[c]!!
+        }
+        node.word = word
+    }
+    val res = mutableListOf<String>()
+    // Start a DFS call from each cell of the board that contains a
+    // child of the root node, which represents the first letter of a
+    // word in the trie.
+    for (r in 0 until board.size) {
+        for (c in 0 until board[0].size) {
+            if (board[r][c] in root.children) {
+                dfs(board, r, c, root.children[board[r][c]], res)
+            }
+        }
+    }
+    return res
+}
+
+fun dfs(board: Array<CharArray>, r: Int, c: Int, node: TrieNode?, res: MutableList<String>) {
+    // If the current node represents the end of a word, add the word to
+    // the result.
+    if (node?.word != null) {
+        res.add(node.word!!)
+        // Ensure the current word is only added once.
+        node.word = null
+    }
+    val temp = board[r][c]
+    // Mark the current cell as visited.
+    board[r][c] = '#'
+    // Explore all adjacent cells that correspond with a child of the
+    // current TrieNode.
+    val dirs = listOf(-1 to 0, 1 to 0, 0 to -1, 0 to 1)
+    for ((dr, dc) in dirs) {
+        val nextR = r + dr
+        val nextC = c + dc
+        if (isWithinBounds(board, nextR, nextC) && board[nextR][nextC] in node?.children.orEmpty()) {
+            dfs(board, nextR, nextC, node?.children?.get(board[nextR][nextC]), res)
+        }
+        // Backtrack by reverting the cell back to its original character.
+        board[r][c] = temp
+    }
+}
+
+fun isWithinBounds(board: Array<CharArray>, r: Int, c: Int): Boolean {
+    return r in board.indices && c in board[0].indices
+}

kotlin/Tries/TrieNode.kt

@@ -1,4 +1,5 @@
 data class TrieNode(
     var isWord: Boolean = false,
+    var word: String? = null,
     val children: HashMap<Char, TrieNode> = hashMapOf()
 )