List Insert

Changing a few next pointers inserts a node at the front or in the middle.

list_insert.c

#include <stdio.h>

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

int main(void) {
    int insertFront = ;
    struct Node second = {20, 0};
    struct Node first = {10, &second};
    struct Node added = {15, 0};
    struct Node *head = &first;

    if (insertFront) {
        added.next = head;
        head = &added;
    } else {
        added.next = first.next;
        first.next = &added;
    }

    printf("head=%d next=%d\n", head->value, head->next->value);
    return 0;
}

#include <stdio.h>

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

int main(void) {
    int insertFront = ;
    struct Node second = {20, 0};
    struct Node first = {10, &second};
    struct Node added = {15, 0};
    struct Node *head = &first;

    if (insertFront) {
        added.next = head;
        head = &added;
    } else {
        added.next = first.next;
        first.next = &added;
    }

    printf("head=%d next=%d\n", head->value, head->next->value);
    return 0;
}

relink Insertion saves the old link before pointing a node at the new neighbor.

head update Inserting at the front changes which node the head pointer references.