请实现一个函数,用来判断一棵二叉树是不是对称的。如果一棵二叉树和它的镜像一样,那么它是对称的。
示例 1:
输入:root = [1,2,2,3,4,4,3]
输出:true
示例 2:
输入:root = [1,2,2,null,3,null,3]
输出:false
限制:
0 <= 节点个数 <= 1000
题解:
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isSymmetric(TreeNode root) {
if (root == null)
return true;
return helper(root.left, root.right);
}
public boolean helper(TreeNode root1, TreeNode root2) {
if (root1 == null && root2 == null)
return true;
if (root1 == null || root2 == null)
return false;
return root1.val == root2.val && helper(root1.left, root2.right) &&
helper(root1.right, root2.left);
}
}