# 思路

以 recursive 遍歷判斷 tree 是否對稱

# 參考程式碼

	/**
	* Definition for a binary tree node.
	* struct TreeNode {
	* int val;
	* TreeNode *left;
	* TreeNode *right;
	* TreeNode() : val(0), left(nullptr), right(nullptr) {}
	* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
	* TreeNode(int x, TreeNode left, TreeNode right) : val(x), left(left), right(right) {}
	* };
	*/

	static auto fast_io = []
	{
	ios::sync_with_stdio(false);
	cout.tie(nullptr);
	cin.tie(nullptr);
	return 0;
	}();

	class Solution {
	public:
	bool isSymmetric(TreeNode* root)
	{
	if (!root) return true;
	return isSymmetric(root->left, root->right);
	}

	bool isSymmetric(TreeNode* l, TreeNode* r)
	{
	if (!l && !r) return true;
	if ((!l && r) \|\| (l && !r) \|\| (l->val != r->val)) return false;
	return isSymmetric(l->left, r->right) && isSymmetric(l->right, r->left);
	}
	};