leetcode--- Symmetric Tree---平衡二叉树

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 ok(TreeNode* left, TreeNode* right)
{
if(left == NULL && right == NULL)
return true;
else if((left == NULL && right != NULL) || (left != NULL && right == NULL))
return false;
else if(left->val != right->val)
return false;
return (ok(left->left, right->right) && ok(left->right, right->left));
}

bool isSymmetric(TreeNode* root)
{
if(root == NULL)
return true;
TreeNode *left, *right;
left = root->left;
right = root->right;
return ok(left, right);
}
};