Merge pull request soapyigu#103 from soapyigu/DFS

Dfs

DFS/NumberofIslands.swift

@@ -2,7 +2,7 @@
  * Question Link: https://leetcode.com/problems/number-of-islands/
  * Primary idea: Classic Depth-first Search, go up, down, left, right four directions
  * 
- * Time Complexity: O((n^2)!), Space Complexity: O(1)
+ * Time Complexity: O(mn), Space Complexity: O(1)
  *
  */
 

DFS/WordSearchII.swift

@@ -0,0 +1,142 @@
+/**
+ * Question Link: https://leetcode.com/problems/word-search-ii/
+ * Primary idea: Classic Depth-first Search, go up, down, left, right four directions 
+ *                  with the help of Trie
+ * 
+ * Time Complexity: O(mn^2), m is the longest length fo a word in words
+ * Space Complexity: O(n^2)
+ *
+ */
+
+class WordSearchII {
+    func findWords(board: [[Character]], _ words: [String]) -> [String] {
+        var res = [String]()
+
+        let m = board.count
+        let n = board[0].count
+
+        let trie = _convertToTrie(words)
+        var visited = [[Bool]](count: m, repeatedValue: Array(count: n, repeatedValue: false))
+
+        for i in 0 ..< m {
+            for j in 0 ..< n {
+                _dfs(board, m, n, i, j, &visited, &res, trie, "")
+            }
+        }
+
+        return res
+    }
+
+    private func _dfs(board: [[Character]], _ m: Int, _ n: Int, _ i: Int, _ j: Int, inout _ visited: [[Bool]], inout _ res: [String], _ trie: Trie, _ str: String) {
+        // beyond matrix
+        guard i >= 0 && i < m && j >= 0 && j < n else {
+            return
+        }
+
+        // check visited
+        guard !visited[i][j] else {
+            return
+        }
+
+        // check is word prefix
+        let str = str + "\(board[i][j])"
+        guard trie.isWordPrefix(str) else {
+            return
+        }
+
+        // check word exist
+        if trie.isWord(str) && !res.contains(str) {
+            res.append(str)
+        }
+
+        // check four directions
+        visited[i][j] = true
+        _dfs(board, m, n, i + 1, j, &visited, &res, trie, str)
+        _dfs(board, m, n, i - 1, j, &visited, &res, trie, str)
+        _dfs(board, m, n, i, j + 1, &visited, &res, trie, str)
+        _dfs(board, m, n, i, j - 1, &visited, &res, trie, str)
+        visited[i][j] = false
+    }
+
+    func _convertToTrie(words: [String]) -> Trie {
+        let trie = Trie()
+
+        for str in words {
+            trie.insert(str)
+        }
+
+        return trie
+    }
+
+
+    class Trie {
+        var root: TrieNode
+
+        init() {
+            root = TrieNode(letter: Character("0"))
+        }
+
+        func insert(word: String) {
+            var node = root
+            var word = [Character](word.characters)
+
+            for i in 0 ..< word.count {
+                let c = word[i]
+
+                if node.children[c] == nil {
+                    node.children[c] = TrieNode(letter: c)
+                }
+
+                node = node.children[c]!
+            }
+
+            node.isEnd = true
+        }
+
+        func isWord(word: String) -> Bool {
+            var node = root
+            var word = [Character](word.characters)
+
+            for i in 0 ..< word.count {
+                let c = word[i]
+
+                if node.children[c] == nil {
+                    return false
+                }
+
+                node = node.children[c]!
+            }
+
+            return node.isEnd
+        }
+
+        func isWordPrefix(prefix: String) -> Bool {
+            var node = root
+            var prefix = [Character](prefix.characters)
+
+            for i in 0 ..< prefix.count {
+                let c = prefix[i]
+
+                if node.children[c] == nil {
+                    return false
+                }
+
+                node = node.children[c]!
+            }
+
+            return true
+        }
+    }
+
+    class TrieNode {
+        var letter: Character
+        var isEnd: Bool
+        var children: [Character:TrieNode]
+
+        init(letter: Character) {
+            self.letter = letter
+            isEnd = false
+            children = [Character:TrieNode]()
+        }
+    }
+}