二叉树转双向链表

class 
{
    public:
        int val;
        Node * left;
        Node * right;
        Node(){val = 0;left = NULL; right = NULL;}
};

Node * process(Node * head)
{
    if( head == NULL ) return NULL;
    Node * l = process(head->left);
    Node * r = process(head->right);
    if( l && r )
    {
        head->left = l;
        head->right = r->right;
        r->right->left = head;
        r->right = l->right;
        l->right = head;
        return r;
    }
    else if( l )
    {
        head->left = l;
        head->right = l->right;
        l->right = head;
        return head;
    }
    else if( r )
    {
        head->left = NULL;
        head->right = r->right;
        r->right = head;
        return r;
    }
    else
    {
        head->right = head;
        head->left = NULL;
        return head;
    }
}

Node * tree2list(Node * head)
{
    if( head == NULL ) return NULL;
    Node *last = process(head);
    head = last->right;
    last->right = NULL;
    return head;
}