feat: add swift implementation to lcof problem: No.54 (#2940)

lcof/面试题54. 二叉搜索树的第k大节点/README.md

@@ -331,6 +331,44 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+/* public class TreeNode {
+*     public var val: Int
+*     public var left: TreeNode?
+*     public var right: TreeNode?
+*     public init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+* }
+*/
+
+class Solution {
+    private var k: Int = 0
+    private var ans: Int = 0
+
+    func kthLargest(_ root: TreeNode?, _ k: Int) -> Int {
+        self.k = k
+        dfs(root)
+        return ans
+    }
+
+    private func dfs(_ root: TreeNode?) {
+        guard let root = root, k > 0 else { return }
+        dfs(root.right)
+        k -= 1
+        if k == 0 {
+            ans = root.val
+            return
+        }
+        dfs(root.left)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题54. 二叉搜索树的第k大节点/Solution.swift

@@ -0,0 +1,33 @@
+/* public class TreeNode {
+*     public var val: Int
+*     public var left: TreeNode?
+*     public var right: TreeNode?
+*     public init(_ val: Int) {
+*         self.val = val
+*         self.left = nil
+*         self.right = nil
+*     }
+* }
+*/
+
+class Solution {
+    private var k: Int = 0
+    private var ans: Int = 0
+
+    func kthLargest(_ root: TreeNode?, _ k: Int) -> Int {
+        self.k = k
+        dfs(root)
+        return ans
+    }
+
+    private func dfs(_ root: TreeNode?) {
+        guard let root = root, k > 0 else { return }
+        dfs(root.right)
+        k -= 1
+        if k == 0 {
+            ans = root.val
+            return
+        }
+        dfs(root.left)
+    }
+}