在第 n 个位置插入一个节点

到目前为止，我们已经研究了在单链表的开头插入一个节点 。但是，大多数情况下，你也希望能够在其他地方插入节点。下面的代码显示了如何编写 insert() 函数以在链接列表中的任何位置插入节点。

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

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

struct Node* insert(struct Node* head, int value, size_t position);
void print_list (struct Node* head);

int main(int argc, char *argv[]) {
  struct Node *head = NULL; /* Initialize the list to be empty */

  /* Insert nodes at positions with values: */
  head = insert(head, 1, 0);
  head = insert(head, 100, 1);
  head = insert(head, 21, 2);
  head = insert(head, 2, 3);
  head = insert(head, 5, 4);
  head = insert(head, 42, 2);

  print_list(head);
  return 0;
}

struct Node* insert(struct Node* head, int value, size_t position) {
  size_t i = 0;
  struct Node *currentNode;

  /* Create our node */
  currentNode = malloc(sizeof *currentNode);
  /* Check for success of malloc() here! */

  /* Assign data */
  currentNode->data = value;

  /* Holds a pointer to the 'next' field that we have to link to the new node.
     By initializing it to &head we handle the case of insertion at the beginning. */
  struct Node **nextForPosition = &head;
  /* Iterate to get the 'next' field we are looking for.
     Note: Insert at the end if position is larger than current number of elements. */
  for (i = 0; i < position && *nextForPosition != NULL; i++) {
      /* nextForPosition is pointing to the 'next' field of the node.
         So *nextForPosition is a pointer to the next node.
         Update it with a pointer to the 'next' field of the next node. */
      nextForPosition = &(*nextForPosition)->next;
  }

  /* Here, we are taking the link to the next node (the one our newly inserted node should
  point to) by dereferencing nextForPosition, which points to the 'next' field of the node
  that is in the position we want to insert our node at.
  We assign this link to our next value. */
  currentNode->next = *nextForPosition;

  /* Now, we want to correct the link of the node before the position of our
  new node: it will be changed to be a pointer to our new node. */
  *nextForPosition = currentNode;

  return head;
}

void print_list (struct Node* head) {
  /* Go through the list of nodes and print out the data in each node */
  struct Node* i = head;
  while (i != NULL) {
    printf("%d\n", i->data);
    i = i->next;
  }
}