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linked-list-print-middle.c

/* print middle element of list; second if even */
/* http://practice.geeksforgeeks.org/problems/finding-middle-element-in-a-linked-list/1 */
/* tl;dr - two pointers, one incrementing by two  */

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

struct Node {
        int data;
        struct Node* next;
};

void push_front(struct Node** head, int data)
{
        struct Node* newnode = (struct Node*) malloc(sizeof(struct Node));
        newnode->data = data;
        newnode->next = (*head);
        (*head) = newnode;
}

void push_end(struct Node** head, int data)
{
        struct Node* newnode = (struct Node*) malloc(sizeof(struct Node));
        newnode->data = data;
        newnode->next = NULL;
        if(*head == NULL) {
                (*head) = newnode;
        } else {
                struct Node* cur = *head;
                while(cur->next != NULL) {
                        cur = cur->next;
                }
                cur->next = newnode;
        }
}

void print_middle(struct Node* head)
{
        struct Node* peek = head;
        struct Node* print = head;

        if(head) {
                while(peek && peek->next) {
                        peek = peek->next->next;
                        print = print->next;
                }
                printf("%d\n", print->data);
        }
}

void print_list(struct Node* lhead)
{
        while(lhead) {
                printf("%d, ", lhead->data);
                lhead = lhead->next;
        }
        printf("END\n");
}

void free_list(struct Node* head)
{
        struct Node* cur = head;
        while((cur = head) != NULL) {
                head = head->next;
                free(cur);
        }
}

int main()
{
        struct Node* start = NULL;

        int num = 6;
        for(int i = 0; i < num; i++) {
                push_end(&start, i);
                print_list(start);
                print_middle(start);
        }
        return 0;
}