Add solution for Symmetric Tree problem in C++

Author: Ahmad-danaf

cpp/0101-symmetric-tree.cpp

@@ -0,0 +1,46 @@
+/*
+    Given the root of a binary tree, this function checks whether the tree is symmetric,
+    i.e., whether it is a mirror of itself around its center.
+
+    Time Complexity: O(n), where n is the number of nodes in the binary tree.
+    Space Complexity: O(n), due to the recursive stack space.
+
+    Approach:
+    - If the root is NULL, return true (base case).
+    - Recursively check if the left subtree of the root is mirrored with the right subtree.
+    - To check if two subtrees are mirrored, compare their left and right children recursively.
+
+    Example:
+    Input: root = [1,2,2,3,4,4,3]
+    Output: true
+
+    Input: root = [1,2,2,null,3,null,3]
+    Output: false
+*/
+
+/**
+ * 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 isSymmetric(TreeNode* root) {
+        return dfs(root, root);
+    }
+
+    bool dfs(TreeNode* left, TreeNode* right) {
+        if (!left && !right)
+            return true;
+        if (!left || !right)
+            return false;
+
+        return left->val == right->val && dfs(left->left, right->right) && dfs(left->right, right->left);
+    }
+};