diff --git a/lcci/04.05.Legal Binary Search Tree/README.md b/lcci/04.05.Legal Binary Search Tree/README.md
index 69df7c795671a..17784c9f040fb 100644
--- a/lcci/04.05.Legal Binary Search Tree/README.md	
+++ b/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)
+ }
+}
+```
+
 
 
 
diff --git a/lcci/04.05.Legal Binary Search Tree/README_EN.md b/lcci/04.05.Legal Binary Search Tree/README_EN.md
index 2d108cdc6e234..1bd098c9c9c19 100644
--- a/lcci/04.05.Legal Binary Search Tree/README_EN.md	
+++ b/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)
+ }
+}
+```
+
 
 
 
diff --git a/lcci/04.05.Legal Binary Search Tree/Solution.swift b/lcci/04.05.Legal Binary Search Tree/Solution.swift
new file mode 100644
index 0000000000000..ebaa5db0b75f0
--- /dev/null
+++ b/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)
+ }
+}
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<p><a href="http://altstyle.alfasado.net">AltStyle</a> によって変換されたページ <a href="https://patch-diff.githubusercontent.com/raw/doocs/leetcode/pull/2657.diff">(-&gt;オリジナル)</a>
/ <label>アドレス: <input type="text" name="naked_post_url" value="https://patch-diff.githubusercontent.com/raw/doocs/leetcode/pull/2657.diff" size="22" /></label> <label>モード: <select name="naked_post_mode">
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<option value="larger-text">文字拡大</option>
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<input type="submit" value="表示" />
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