Answer: Lists

Question: Write a function to insert an element onto the front of a doubly linked list of nodes with the following type definition.

    struct ListNode {
        int             data;
        ListNode        *next; // pointer to next elt in list
        ListNode        *prev; // pointer to previous elt in list
    };

Answer:

ListNode*
insertFront(ListNode *old_head, int key) {
    ListNode *new_head = new ListNode;
    new_head->data = key;
    new_head->prev = NULL;
    new_head->next = old_head;
    if(old_head) old_head->prev = new_head;
    return new_head;
}