Created Leetcode's Symmetric tree readme file with it solution in cpp

Symmetric Tree/README.md

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+<h2><a href="https://leetcode.com/problems/symmetric-tree/">Symmetric Tree</a></h2><hr><div><p>Given the <code>root</code> of a binary tree, <em>check whether it is a mirror of itself</em> (i.e., symmetric around its center).</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/symtree1.jpg" style="width: 354px; height: 291px;">
+<pre><strong>Input:</strong> root = [1,2,2,3,4,4,3]
+<strong>Output:</strong> true
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/symtree2.jpg" style="width: 308px; height: 258px;">
+<pre><strong>Input:</strong> root = [1,2,2,null,3,null,3]
+<strong>Output:</strong> false
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[1, 1000]</code>.</li>
+	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<strong>Follow up:</strong> Could you solve it both recursively and iteratively?</div>

Symmetric Tree/symmetric-tree.cpp

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+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isSameTree(TreeNode* p, TreeNode* q){
+        if(p==NULL && q==NULL)
+            return true;
+        if(p==NULL || q==NULL)
+            return false;
+        if(p->val!=q->val)
+            return false;
+    return isSameTree(p->left,q->right)&&isSameTree(p->right,q->left);
+}
+
+    bool isSymmetric(TreeNode* root){
+        if(root==NULL)
+            return true;
+    return isSameTree(root->left,root->right);
+    }
+};