判断一颗二叉树是不是对称的

boolean isSymmetrical(TreeNode pRoot)
{
if(pRoot == null){
return true;
}
return isMirror(pRoot.left,pRoot.right);
}
public static boolean isMirror(TreeNode t1, TreeNode t2){

if(t1 == null && t2 == null){
return true;
}
if(t1 == null ||t2 == null)
return false;
if(t1.val != t2.val){
return false;
}
return (isMirror(t1.left,t2.right)) && (isMirror(t1.right,t2.left));
}