101. Symmetric Tree

https://leetcode.com/problems/symmetric-tree/description/
解题思路：

1. node1. left == node2.right && node1.right == node2.left

代码：
class Solution {
public boolean isSymmetric(TreeNode root) {

    return preorder(root, root);
}
public boolean preorder(TreeNode node1, TreeNode node2){
    if(node1 == null && node2 == null) return true;
    if(node1 != null && node2 != null && node1.val == node2.val)
        return preorder(node1.left, node2.right) && preorder(node1.right, node2.left);
    return false;
}

}