Update 0101.对称二叉树.md

Author: EnzoSeason

problems/0101.对称二叉树.md

@@ -754,23 +754,77 @@ func isSymmetric3(_ root: TreeNode?) -> Bool {
 
 ## Scala
 
-递归：
+> 递归：
 ```scala
-object Solution {
+object Solution { 
   def isSymmetric(root: TreeNode): Boolean = {
     if (root == null) return true // 如果等于空直接返回true
+    
     def compare(left: TreeNode, right: TreeNode): Boolean = {
-      if (left == null && right == null) return true // 如果左右都为空，则为true
-      if (left == null && right != null) return false // 如果左空右不空，不对称，返回false
-      if (left != null && right == null) return false // 如果左不空右空，不对称，返回false
+      if (left == null && right == null) true // 如果左右都为空，则为true
+      else if (left == null && right != null) false // 如果左空右不空，不对称，返回false
+      else if (left != null && right == null) false // 如果左不空右空，不对称，返回false
       // 如果左右的值相等，并且往下递归
-      left.value == right.value && compare(left.left, right.right) && compare(left.right, right.left)
+      else left.value == right.value && compare(left.left, right.right) && compare(left.right, right.left)
     }
+    
     // 分别比较左子树和右子树
     compare(root.left, root.right)
   }
 }
 ```
+> 迭代 - 使用栈
+```scala
+object Solution {
+
+  import scala.collection.mutable
+
+  def isSymmetric(root: TreeNode): Boolean = {
+    if (root == null) return true
+
+    val cache = mutable.Stack[(TreeNode, TreeNode)]((root.left, root.right))
+    
+    while (cache.nonEmpty) {
+      cache.pop() match {
+        case (null, null) =>
+        case (_, null) => return false
+        case (null, _) => return false
+        case (left, right) =>
+          if (left.value != right.value) return false
+          cache.push((left.left, right.right))
+          cache.push((left.right, right.left))
+      }
+    }
+    true
+  }
+}
+```
+> 迭代 - 使用队列
+```scala
+object Solution {
+
+  import scala.collection.mutable
+
+  def isSymmetric(root: TreeNode): Boolean = {
+    if (root == null) return true
+
+    val cache = mutable.Queue[(TreeNode, TreeNode)]((root.left, root.right))
+    
+    while (cache.nonEmpty) {
+      cache.dequeue() match {
+        case (null, null) =>
+        case (_, null) => return false
+        case (null, _) => return false
+        case (left, right) =>
+          if (left.value != right.value) return false
+          cache.enqueue((left.left, right.right))
+          cache.enqueue((left.right, right.left))
+      }
+    }
+    true
+  }
+}
+```
 
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">