Symmetric Tree [LeetCode]

/**
* 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 isSymmetric(TreeNode *root) {
// Note: The Solution object is instantiated only once and is reused by each test case.
if(root == NULL)
return true;

vector<TreeNode*> layer;
layer.push_back(root);
while(true) {
//determine if the tree is symetric
for(int i = 0; i < layer.size()/2; i ++ ) {
//layer[i] compare layer[layer.size() - 1 - i]
int right_idx = layer.size() - 1 - i;
if((layer[i]->left == NULL && layer[right_idx]->right == NULL ||
(layer[i]->left != NULL && layer[right_idx]->right != NULL &&
layer[i]->left->val == layer[right_idx]->right->val)) &&
(layer[i]->right == NULL && layer[right_idx]->left == NULL ||
(layer[i]->right != NULL && layer[right_idx]->left != NULL &&
layer[i]->right->val == layer[right_idx]->left->val)))
continue;
else
return false;
}

if(layer.size() % 2 != 0) {
int middle = layer.size() / 2;
if(layer[middle]->left == NULL && layer[middle]->right == NULL ||
(layer[middle]->left != NULL && layer[middle]->right != NULL &&
layer[middle]->left->val == layer[middle]->right->val)){
//do nothing
}else{
return false;
}
}

//get node for next layer
vector<TreeNode*> new_layer;
for(auto node : layer) {
if(node->left != NULL)
new_layer.push_back(node->left);
if(node->right != NULL)
new_layer.push_back(node->right);
}
if(new_layer.size() == 0)
break;

layer = new_layer;
}

return true;
}
};

class Solution {
public:
bool isSubSymmetric(TreeNode * left, TreeNode * right) {
if(left == NULL && right == NULL)
return true;
else if(left != NULL && right == NULL || (right != NULL && left == NULL))
return false;
else if(left->val != right->val)
return false;
else
return isSubSymmetric(left->left, right->right) && isSubSymmetric(left->right, right->left);
}
bool isSymmetric(TreeNode *root) {
// Note: The Solution object is instantiated only once and is reused by each test case.
if(root == NULL)
return true;

return isSubSymmetric(root->left, root->right);
}
};