Update 二叉树相关.md

blogs/Algorithm/剑指 Offer/二叉树相关.md

@@ -425,3 +425,62 @@ public class Codec {
 }
 ```
 
+#### [28. 对称的二叉树](https://leetcode-cn.com/problems/dui-cheng-de-er-cha-shu-lcof/)
+
+```java
+class Solution {
+
+    public boolean isSymmetric(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        return helper(root.left, root.right);
+    }
+
+    private boolean helper(TreeNode root1, TreeNode root2) {
+        if (root1 == null && root2 == null) {
+            return true;
+        }
+        if (root1 == null || root2 == null) {
+            return false;
+        }
+        if (root1.val != root2.val) {
+            return false;
+        }
+        return helper(root1.left, root2.right) && helper(root1.right, root2.left);
+    }
+}
+```
+
+```java
+class Solution {
+
+    public boolean isSymmetric(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        Deque<TreeNode> queue = new LinkedList<>();
+        queue.addFirst(root.left);
+        queue.addLast(root.right);
+        while (!queue.isEmpty()) {
+            TreeNode first = queue.removeFirst();
+            TreeNode last = queue.removeLast();
+            if (first == null && last == null) {
+                continue;
+            }
+            if (first == null || last == null) {
+                return false;
+            }
+            if (first.val != last.val) {
+                return false;
+            }
+            queue.addFirst(first.right);
+            queue.addFirst(first.left);
+            queue.addLast(last.left);
+            queue.addLast(last.right);
+        }
+        return true;
+    }
+}
+```
+