对称的二叉树

Description：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 {
    boolean isSymmetrical(TreeNode root)
    {
        if (root == null)
            return true;
        return backtrack(root.left, root.right);
    }
    
    public boolean backtrack(TreeNode left, TreeNode right) {
        if (left == null) 
            return right == null;
        if (right == null)
            return false;
        if (left.val != right.val) 
            return false;
        return backtrack(left.right, right.left) && backtrack(left.left, right.right);
    }
}