word search ii solution

Author: dolphinflow86

word-search-ii/dolphinflow86.py

@@ -0,0 +1,59 @@
+# M, N are dimensions of the board, W is the number of words, and L is the maximum length of a word.
+# TC: O(W * L + M * N * 3^(L - 1)) - building Trie takes O(W * L); backtracking explores at most 3 directions after first step up to depth L
+# SC: O(W * L) - Trie storage for all words and recursion stack depth up to L
+
+from typing import List
+
+
+class TrieNode:
+
+    def __init__(self):
+        self.children = {}
+        self.word = None
+
+
+class Solution:
+
+    def findWords(self, board: List[List[str]], words: List[str]) -> List[str]:
+        root = TrieNode()
+        for word in words:
+            node = root
+            for char in word:
+                if char not in node.children:
+                    node.children[char] = TrieNode()
+                node = node.children[char]
+            node.word = word
+
+        rows, cols = len(board), len(board[0])
+        result = []
+
+        def dfs(r: int, c: int, parent: TrieNode):
+            char = board[r][c]
+            curr_node = parent.children.get(char)
+            if not curr_node:
+                return
+
+            if curr_node.word:
+                result.append(curr_node.word)
+                curr_node.word = None
+
+            board[r][c] = "#"  # mark visited
+
+            for dr, dc in ((-1, 0), (1, 0), (0, -1), (0, 1)):
+                nr, nc = r + dr, c + dc
+                if 0 <= nr < rows and 0 <= nc < cols and board[nr][nc] != "#":
+                    if board[nr][nc] in curr_node.children:
+                        dfs(nr, nc, curr_node)
+
+            board[r][c] = char  # backtrack
+
+            # Prune leaf nodes to accelerate search
+            if not curr_node.children:
+                del parent.children[char]
+
+        for r in range(rows):
+            for c in range(cols):
+                if board[r][c] in root.children:
+                    dfs(r, c, root)
+
+        return result