feat(ml):$101.symmetric-tree.md (#449)

Author: ztianming

problems/101.symmetric-tree.md

@@ -98,7 +98,84 @@ return True
 > 其实更像本题一点的话应该是从中间分别向两边扩展 😂
 
 ## 代码
+代码支持：C++, Java, Python3
+
+C++ Code:
+```c++
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    bool isSymmetric(TreeNode* root) {
+        return root==NULL?true:recur(root->left, root->right);
+    }
+
+    bool recur(TreeNode* l, TreeNode* r)
+    {
+        if(l == NULL && r==NULL)
+        {
+            return true;
+        }
+        // 只存在一个子节点 或者左右不相等 
+        if(l==NULL || r==NULL || l->val != r->val)
+        {
+            return false;
+        }
+
+        return recur(l->left, r->right) && recur(l->right, r->left);
+    }
+};
+```
+
+
+Java Code:
+```java
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public boolean isSymmetric(TreeNode root) {
+        if(root == null)
+        {
+            return true;
+        }
+        else{
+            return recur(root.left, root.right);
+        }
+        // return root == null ? true : recur(root.left, root.right);
+    }
+
+    public boolean recur(TreeNode l, TreeNode r)
+    {
+        if(l == null && r==null)
+        {
+            return true;
+        }
+        // 只存在一个子节点 或者左右不相等 
+        if(l==null || r==null || l.val != r.val)
+        {
+            return false;
+        }
+
+        return recur(l.left, r.right) && recur(l.right, r.left);
+    }
+}
+```
 
+Python3 Code:
 ```py
 
 class Solution: