Symmetric Tree leetcode

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

和same tree那道题思路一样, 终止条件还是1.左子树为空 右子树不为空 2. 右子树为空 左子树不为空 3. 左子树的值和右子树不同
一颗树对称其实就是看左右子树是否对称，一句话就是左同右，右同左，结点是对称的相等。
算法的时间复杂度是树的遍历O(n)，空间复杂度同样与树遍历相同是O(logn)

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return helper(root.left, root.right);
    }
    public boolean helper(TreeNode node1, TreeNode node2) {
        if (node1 == null && node2 == null) {
            return true;
        }
        if (node1 == null || node2 == null) {
            return false;
        }
        if (node1.val != node2.val) {//if true 继续递归
            return false;
        }
        return helper(node1.left, node2.right) && helper(node2.left, node1.right);
    }
}