feat: add swift implementation to lcof problem: No.33 (doocs#2903)

Author: klever34

lcof/面试题33. 二叉搜索树的后序遍历序列/README.md

@@ -269,6 +269,36 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    private var postorder: [Int] = []
+
+    func verifyPostorder(_ postorder: [Int]) -> Bool {
+        self.postorder = postorder
+        return dfs(0, postorder.count - 1)
+    }
+
+    private func dfs(_ l: Int, _ r: Int) -> Bool {
+        if l >= r {
+            return true
+        }
+        let rootValue = postorder[r]
+        var i = l
+        while i < r && postorder[i] < rootValue {
+            i += 1
+        }
+        for j in i..<r {
+            if postorder[j] < rootValue {
+                return false
+            }
+        }
+        return dfs(l, i - 1) && dfs(i, r - 1)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题33. 二叉搜索树的后序遍历序列/Solution.swift

@@ -0,0 +1,25 @@
+class Solution {
+    private var postorder: [Int] = []
+
+    func verifyPostorder(_ postorder: [Int]) -> Bool {
+        self.postorder = postorder
+        return dfs(0, postorder.count - 1)
+    }
+
+    private func dfs(_ l: Int, _ r: Int) -> Bool {
+        if l >= r {
+            return true
+        }
+        let rootValue = postorder[r]
+        var i = l
+        while i < r && postorder[i] < rootValue {
+            i += 1
+        }
+        for j in i..<r {
+            if postorder[j] < rootValue {
+                return false
+            }
+        }
+        return dfs(l, i - 1) && dfs(i, r - 1)
+    }
+}