Check is Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1

   / \

  2   2

 / \ / \

3  4 4  3

But the following is not:

    1

   / \

  2   2

   \   \

   3    3

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

       bool SymmetricUtil(TreeNode *n1, TreeNode *n2) {

              if(n1 == nullptr && n2 == nullptr) return true;

              if(n1 == nullptr || n2 == nullptr) return false;

              if(n1->val != n2->val) return false;

              return SymmetricUtil(n1->left, n2->right) && SymmetricUtil(n1->right, n2->left);

       }

       bool isSymmetric(TreeNode *root) {

              // Start typing your C/C++ solution below

              // DO NOT write int main() function

              if(root == nullptr) return true;

              return SymmetricUtil(root->left, root->right);

       }

};