Linked list

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#include <stdio.h>
#include <stdlib.h>
#include "list.h"


// displayList()
// -------------
// Given a list, print its elements.
//
void displayList(listNode * list) {
    printf("list: ");
    while (list != NULL) {
        printf("%d ", list->value);
        list = list->next;
    }
    printf("%p\n", list);
    return;
}


// push()
// ------
// Given a pointer to the head node of a list and a value,
// add a new node with that value to the head of the list.
//
void push(listNode ** listPtr, int value) {
    listNode * newNodePtr = (listNode *) malloc(sizeof(listNode));
    newNodePtr->value = value;
    newNodePtr->next = *listPtr;
    *listPtr = newNodePtr;
    return;
}


// pop()
// -----
// Given a pointer to the head node of a list, remove the head node,
// free the head node's memory, and return the head node's value.
// Return 0 if the list is empty.
//
int pop(listNode ** listPtr) {
    int value = 0;
    listNode * tempPtr;
    if (*listPtr) {
        tempPtr = *listPtr;
        value = tempPtr->value;
        *listPtr = tempPtr->next;
        free(tempPtr);
    }
    return value;
}


// destroyList()
// -------------
// Given a pointer to the head of a list, remove and free the memory
// of all elements of the list.
//
void destroyList(listNode ** listPtr) {
    listNode * temp = *listPtr;
    while (temp != NULL) {
        *listPtr = (*listPtr)->next;
        free(temp);
        temp = *listPtr;
    }
    return;
}


// count()
// -------
// Given a pointer to the head node of a list and a value, return an 
// integer representing the number of times that value appears in the 
// list.
//
int count(listNode * listPtr, int value) {
    int c = 0;
    while (listPtr != NULL) {
        if (value == listPtr->value) c++;
        listPtr = listPtr->next;
    }
    return c;
} 


// insert()
// --------
// Given a pointer to the head node of a list and a value, add a new
// node with the value to the tail of the list. If the list is
// empty, the new node becomes the head node.
//
void insert(listNode ** listPtr, int value) {

    // Get the first node in the list.
    listNode * temp = *listPtr;

    // Create the new node to be appended.
    listNode * newNodePtr = (listNode *) malloc(sizeof(listNode));
    newNodePtr->value = value;
    newNodePtr->next = NULL;

    // If the list is empty, the new node becomes the head node.
    if (temp == NULL) {
        *listPtr = newNodePtr;
        return;
    }

    // Otherwise, traverse the list to find the last node.
    while (temp->next != NULL) temp = temp->next;

    // The node pointer in the last node in the list must be the
    // pointer to the new, appended node.
    temp->next = newNodePtr;
}


// reverse()
// ---------
// Given a pointer to the head node of a list, reverse the order of
// nodes in the list by relinking the existing nodes (do not allocate
// or free any memory).
//
void reverse(listNode ** listPtr) {

    // Get the first node in the list.
    listNode * curr = *listPtr;

    // If the list is empty, don't do anything to it..
    if (curr == NULL) return;

    // Otherwise, create two more pointers:
    // - `next` for storing the next node
    // - `prev` for storing the previous node
    // 
    // We store the pointer to the next node in `next`, then set the 
    // current node's node pointer to the pointer to the previous node.
    // Since the first node in the list will become the last node in
    // the reversed list, the first node's node pointer should be null.
    // Then, we set the previous node to the current node, and the
    // current node to the next node, iterating until the current node
    // is null. When the loop has completed, set the new head of the 
    // list to the final value of the previous node.
    listNode * prev = NULL;
    listNode * next = NULL;

    while (curr != NULL) {
        next = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next;
    }

    *listPtr = prev;
}


// removeAll()
// -----------
// Given a pointer to the head node of a list and a value, remove
// all nodes in the list that have that value, freeing the memory
// space for those nodes (and re-linking the surviving nodes). Must 
// work properly when the value is at the head or the tail of the list.
//
void removeAll(listNode ** listPtr, int value) {

    // Get the first node in the list.
    listNode * curr = *listPtr;

    // If the list is empty, don't do anything to it.
    if (curr == NULL) return;

    // Remove all contiguous leading nodes containing the value.
    while (curr != NULL && value == curr->value) {
        *listPtr = curr->next;
        free(curr);
        curr = *listPtr;
    }

    // If we're not done, we'll need to keep track of the previous node
    // while processing the rest of the list.
    listNode * prev;

    while (curr != NULL) {

        // Find the next occurrence of the value.
        while (curr != NULL && value != curr->value) {
            prev = curr;
            curr = curr->next;
        }

        // If we haven't found any other occurrences of the value, return.
        if (curr == NULL) return;

        // If we have found an occurrence of the value, relink the previous
        // node to the next node, then free the current node's memory and 
        // assign the (new) next node to its name.
        prev->next = curr->next;
        free(curr);
        curr = prev->next;
    }

}