[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:

But the following is not:

Note:
Bonus points if you could solve it both recursively and iteratively.

递归，保存左右两个节点，然后判断leftNode->left和rightNode->right，以及leftNode->right和rightNode->left。如此不断递归

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool check(TreeNode *leftNode, TreeNode *rightNode)
13     {
14         if (leftNode == NULL && rightNode == NULL)
15             return true;
16             
17         if (leftNode == NULL || rightNode == NULL)
18             return false;
19             
20         return leftNode->val == rightNode->val && check(leftNode->left, rightNode->right) && 
21             check(leftNode->right, rightNode->left);
22     }
23     
24     bool isSymmetric(TreeNode *root) {
25         // Start typing your C/C++ solution below
26         // DO NOT write int main() function
27         if (root == NULL)
28             return true;
29             
30         return check(root->left, root->right);
31     }
32 };