101. Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree 

[1,2,2,3,4,4,3]

 is symmetric:

1
/ \
2   2
/ \ / \
3  4 4  3

But the following 

[1,2,2,null,3,null,3]

 is not:

1
/ \
2   2
\   \
3    3

考虑递归，左子树的左枝 ==右子树的右枝 && 左子树的右枝 == 右子树的左枝

/**
* 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 isMiror(root->left, root->right);
}
bool isMiror(TreeNode *n1, TreeNode *n2) {
if (!n1 && !n2) return true;
if (!(n1 && n2)) return false;
if (n1->val != n2->val) return false;
return isMiror(n1->left, n2->right) && isMiror(n1->right, n2->left);
}
};