LeetCode_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

根开始，如果根节点的左右不对称，则false，否则，看根节点的左右子树是否对称。

代码：（AC）

/**
* 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 left_right_Symmetric(TreeNode* pleft,TreeNode* pright)
{
if(pleft==NULL&&pright==NULL) return true;
if(pleft==NULL&&pright!=NULL)return false;
if(pleft!=NULL&&pright==NULL)return false;
if(pleft->val!=pright->val) return false;
return left_right_Symmetric(pleft->left,pright->right)&&left_right_Symmetric(pleft->right,pright->left);
}
public:
bool isSymmetric(TreeNode* root) {
if(root == NULL) return true;
if(root->left!=NULL&&root->right==NULL) return false;
if(root->left==NULL&&root->right!=NULL) return false;
if(root->left!=NULL&&root->right!=NULL&&root->left->val!=root->right->val)return false;
else return left_right_Symmetric(root->left,root->right);
}
};

错误代码：没有理解对称树的概念。（WA）

/**
* 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;
if(root->left==NULL&&root->right!=NULL) return false;
else if(root->left!=NULL&&root->right==NULL)return false;
else if(root->left==NULL&&root->right==NULL) return true;
else if(root->left!=NULL&&root->right!=NULL&&root->left->val!=root->right->val) return false;
else if(root->left!=NULL&&root->right!=NULL&&root->left->val==root->right->val)
return isSymmetric(root->left)&&isSymmetric(root->right);
}
};