swap list

swap_list.c

#include <stdio.h>
#include <stddef.h>

typedef struct _List {
  struct _List *next;
  int val;
} List;

enum {
  SWAPSUCCESS,
  EMPTYLIST,
  INVALIDSWAP,
};

int swap_list(List **head, List *a, List *b) {
  // a, b identical
  if (a == b) { return SWAPSUCCESS; }
  // a, b not exist
  if (a == NULL || b == NULL) { return INVALIDSWAP; }
  // empty list
  if (*head == NULL) { return EMPTYLIST; }

  List dummy;
  dummy.next = *head;
  List *p1 = &dummy;
  while(p1->next) {
    if (p1->next == a || p1->next == b) {
      // find swap point
      List *target = (p1->next == a)? b : a;
      List *p2 = p1->next;
      while(p2->next) {
        if (p2->next == target) {
          // find swap point, do swap
          p2->next = p1->next;
          p1->next = target;
          target = target->next;
          p1->next->next = p2->next->next; 
          p2->next->next = target;
          *head = dummy.next;
          return SWAPSUCCESS;
        }
        p2 = p2->next;
      }
      return INVALIDSWAP;
    }
    p1 = p1->next;
  }
  return INVALIDSWAP;
}

void
printList(List *p)
{
  while(p) {
    printf("%d->", p->val);
    p = p->next;
  }
  printf("NULL\n");
}

int main(int argc, char *argv[])
{
  List l[10];
  // build list
  for (int i = 0; i < 10; ++i) {
    l[i].val = i;
    l[i].next = &(l[i+1]);
  }
  l[9].next = NULL;
  //do swap
  List *p = l;
  printList(p);
  swap_list(&p, &l[0], &l[9]);
  printList(p);
  swap_list(&p, &l[3], &l[5]);
  printList(p);
  return 0;
}