链表的基本操作（Basic Operations on a Linked List）

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

//creating the basic structure of a node of a Linked List
struct node
{
    int data;
    struct node * next;
};

/* Defining a function to create a new list.
The return type of the function is struct node, because we will return the head node
The parameters will be the node, passed from the main function.
The second parameter will be the data element.
*/
struct node * createList(struct node *head,int number)
{
    //creating a temporary node for our initialization purpose
    
    //the below line allocates a space in memory that can hold the data in a node.
    struct node *temp = (struct node *)malloc(sizeof(struct node));
    
    //assigning the number to data
    temp -> data = number;
    
    //assigning the next of this node to NULL, as a new list is formed.
    temp -> next = NULL;
    
    //now since we have passed head as the parameter, we need to return it.
    head = temp;
    return head;
}

/*
Defining a case when we need to element in the Linked List.
Here 2 cases can arise:-
    -The Linked List can be empty, then we need to create a new list
    -The Linked List exists, we need to add the element
*/
struct node * addElement(struct node *head, int number)
{
    if(head == NULL)
        head = createList(head, number);   // we can directly call the above function
    else
    {
        // now this is a case when we need to add an element to an existing list.
        
        //Creating a new node and assigning values
        struct node * temp = (struct node*)malloc(sizeof(struct node));
        temp -> data = number;
        temp -> next = NULL;
        
        //Now we have created the node but, we need to insert it at the right place.
        //A Linked List terminates when we encounter a NULL
        //Let us traverse the List using another temporary variable.
        //We will point it to the start
        struct node * temp2 = head;
        
        //The limiting condition of the while loop "until temp2‘s NEXT is not equal to NULL
        //This will happen only in case of the last node of the list.
        while(temp2 -> next != NULL)
        {
            // We need to go to the next node
            temp2 = temp2 -> next;
        }
        
        //Now temp2 points at the last node of the list.
        //We can add the node at this position now.
        
        temp2 -> next = temp;  // the number is added to the end of the List.
    }
    
    return head; // because we only need the HEAD pointer for a Linked List.
}

/*
A function to print the Linked List.
The return type of this function will be void, as we do not need to return anything.
We just need the HEAD as the parameter, to traverse the Linked List.
*/
void printList(struct node * head)
{
    // The terminating point of a Linked List is defined when we encounter a NULL
    while(head != NULL)
    {
        printf("%d ",head->data);
        
        //now we need to move to the next element
        head = head->next;
    }
}

//DEFINING THE MAIN FUNCTION

int main(void)
{
    struct node * listHEAD = NULL;
    
    listHEAD = createList(listHEAD, 42);  // creating a new List with starting number 42
    
    //adding a few number to the list
    listHEAD = addElement(listHEAD, 31);
    listHEAD = addElement(listHEAD, 98);
    listHEAD = addElement(listHEAD, 100);
    
    //finally printing our Linked List
    printList(listHEAD);
    
    return 0;
}