feat: add swift implementation to lcci problem: No.04.05

lcci/04.05.Legal Binary Search Tree/README.md

@@ -291,6 +291,47 @@ public class Solution {
 }
 ```
 
+```swift
+/* class TreeNode {
+*    var val: Int
+*    var left: TreeNode?
+*    var right: TreeNode?
+*
+*    init(_ val: Int) {
+*        self.val = val
+*        self.left = nil
+*        self.right = nil
+*    }
+* }
+*/
+
+class Solution {
+    private var prev: TreeNode?
+
+    func isValidBST(_ root: TreeNode?) -> Bool {
+        return dfs(root)
+    }
+
+    private func dfs(_ root: TreeNode?) -> Bool {
+        guard let root = root else {
+            return true
+        }
+
+        if !dfs(root.left) {
+            return false
+        }
+
+        if let prev = prev, prev.val >= root.val {
+            return false
+        }
+
+        prev = root
+
+        return dfs(root.right)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- end -->

lcci/04.05.Legal Binary Search Tree/README_EN.md

@@ -328,6 +328,47 @@ public class Solution {
 }
 ```
 
+```swift
+/* class TreeNode {
+*    var val: Int
+*    var left: TreeNode?
+*    var right: TreeNode?
+*
+*    init(_ val: Int) {
+*        self.val = val
+*        self.left = nil
+*        self.right = nil
+*    }
+* }
+*/
+
+class Solution {
+    private var prev: TreeNode?
+
+    func isValidBST(_ root: TreeNode?) -> Bool {
+        return dfs(root)
+    }
+
+    private func dfs(_ root: TreeNode?) -> Bool {
+        guard let root = root else {
+            return true
+        }
+
+        if !dfs(root.left) {
+            return false
+        }
+
+        if let prev = prev, prev.val >= root.val {
+            return false
+        }
+
+        prev = root
+
+        return dfs(root.right)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- end -->

lcci/04.05.Legal Binary Search Tree/Solution.swift

@@ -0,0 +1,38 @@
+/* class TreeNode {
+*    var val: Int
+*    var left: TreeNode?
+*    var right: TreeNode?
+*
+*    init(_ val: Int) {
+*        self.val = val
+*        self.left = nil
+*        self.right = nil
+*    }
+* }
+*/
+
+class Solution {
+    private var prev: TreeNode?
+
+    func isValidBST(_ root: TreeNode?) -> Bool {
+        return dfs(root)
+    }
+
+    private func dfs(_ root: TreeNode?) -> Bool {
+        guard let root = root else {
+            return true
+        }
+
+        if !dfs(root.left) {
+            return false
+        }
+
+        if let prev = prev, prev.val >= root.val {
+            return false
+        }
+
+        prev = root
+
+        return dfs(root.right)
+    }
+}