这题的思路也是递归（递归在二叉树中很重要哦）。 

class Solution {    public boolean isSymmetric(TreeNode root) {        if (root ==null){            return true;        }        return symmetric (root.right,root.left);    }    private boolean symmetric(TreeNode right,TreeNode left){        if(left == null && right == null) return true;        if(left == null || right == null) return false;        return (left.val==right.val)&&symmetric(left.left,right.right)&&symmetric(left.right,right.left);    }}