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