101. 对称二叉树
给你一个二叉树的根节点 root ， 检查它是否轴对称。

 

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
 

提示：

树中节点数目在范围 [1, 1000] 内
-100 <= Node.val <= 100

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
 public static boolean isSymmetric(TreeNode root) {


        return isSymmetric(root.left, root.right);
    }

    public static boolean isSymmetric(TreeNode p, TreeNode q) {
        if (p == null && q == null) {
            return true;
        }

        if (p == null || q == null) {
            return false;
        }

        if (p.val != q.val) {
            return false;
        }

        return isSymmetric(p.left, q.right) && isSymmetric(p.right, q.left);

    }
}