Symmetric Tree.txt

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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solution:
//递归解决
struct TreeNode {
  int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

bool isSymmetric_aux(TreeNode *node1, TreeNode *node2)
{
	if(node1== NULL && node2 == NULL)
		return true;
	if(node1 == NULL || node2 == NULL)
		return false;
	return node1->val == node2->val && isSymmetric_aux(node1->left, node2->right) && isSymmetric_aux(node1->right, node2->left);
}

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