LeetCode: Symmetric Tree

Title:

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 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) {
if (root == NULL)
return true;
return fun(root->left,root->right);

}
bool fun(TreeNode* p1, TreeNode* p2){
if (!p1 && !p2 )
return true;
if (!p1 || !p2 )
return false;
return (p1->val == p2->val) && fun(p1->left,p2->right) && fun(p1->right,p2->left);

}
};

/**
* 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) {
if (root == NULL)
return true;
stack<TreeNode* > s;
s.push(root->left);
s.push(root->right);
while (!s.empty()){
TreeNode* p1 = s.top();
s.pop();
TreeNode* p2 = s.top();
s.pop();
if (!p1 && !p2)
continue;
if (!p1 || !p2)
return false;
if (p1->val != p2->val)
return false;
s.push(p1->left);
s.push(p2->right);
s.push(p1->right);
s.push(p2->left);
}
return true;
}

};