对称的二叉树

请实现一个函数，用来判断一颗二叉树是不是对称的。注意，如果一个二叉树同此二叉树的镜像是同样的，定义其为对称的。

/*public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}*/public class Solution {    public boolean isSymmetrical(TreeNode pRoot)    {        if (pRoot == null) return true;        return leftIsSameright(pRoot.left, pRoot.right);    }    private boolean leftIsSameright(TreeNode left, TreeNode right) {        if (left==null&&right==null) return true;        if (left==null&&right!=null) return false;        if (left!=null&&right==null) return false;        if (left.val!=right.val) return false;        return (leftIsSameright(left.right,right.left)&&(leftIsSameright(left.left,right.right)));    }}