Объединение списков

#include <stdio.h>
#include <stdlib.h>
 
struct listNode {
    char data; 
    struct listNode *nextPtr; 
}; 
 
typedef struct listNode ListNode; 
typedef ListNode* ListNodePtr; 
 
 
void insert( ListNodePtr *sPtr, char value );
int isEmpty( ListNodePtr sPtr );
void printList( ListNodePtr currentPtr );
void instructions( void );
ListNodePtr MergeLists(ListNodePtr list1, ListNodePtr list2);
 
int main( void )
{
    ListNodePtr startPtr = NULL; 
    ListNodePtr startPtr2 = NULL;
    int choice; 
    char item;
    instructions(); 
    printf( "? " );
    scanf( "%d", &choice );
    /* loop while user does not choose 3 */
    while ( choice != 4 ) {
        switch ( choice ) {
            case 1:
                printf( "Enter a character: " );
                scanf( "\n%c", &item );
                insert( &startPtr, item ); /* insert item in list */
                printList( startPtr );
                break;
                case 2:
                    printf( "Enter a character2sp: " );
                    scanf( "\n%c", &item );
                    insert( &startPtr2, item ); /* insert item in list */
                    printList(startPtr2);
                    break;
                case 5:
                        MergeLists(startPtr,startPtr2); 
                    break;
            default:
                printf( "Invalid choice.\n\n" );
                instructions();
            break;
        } /* end switch */
        
        printf( "? " );
        scanf( "%d", &choice );
    } /* end while */
    printf( "End of run.\n" );
    return 0; /* indicates successful termination */
} /* end main */
 
/* display program instructions to user */
void instructions( void )
{
    printf( "Enter your choice:\n"
    "1 to insert an element into the list.\n"
    "2 to insert an element into the list2.\n"
    "4 to end.\n" );
} /* end function instructions */
 
/* Insert a new value into the list in sorted order */
void insert( ListNodePtr *sPtr, char value )
{
    ListNodePtr newPtr; /* pointer to new node */
    ListNodePtr previousPtr; /* pointer to previous node in list */
    ListNodePtr currentPtr; /* pointer to current node in list */
 
    newPtr = malloc( sizeof( ListNode ) ); /* create node */
    if ( newPtr != NULL ) { /* is space available */
        newPtr->data = value; /* place value in node */
        newPtr->nextPtr = NULL; /* node does not link to another node */
        previousPtr = NULL;
        currentPtr = *sPtr;
        /* loop to find the correct location in the list */
        while ( currentPtr != NULL && value > currentPtr->data ) {
            previousPtr = currentPtr;
            /* walk to ... */
            currentPtr = currentPtr->nextPtr; /* ... next node */
        } /* end while */   
        /* insert new node at beginning of list */
        if ( previousPtr == NULL ) {
            newPtr->nextPtr = *sPtr;
            *sPtr = newPtr;
        } /* end if */
        else { /* insert new node between previousPtr and currentPtr */
            previousPtr->nextPtr = newPtr;
            newPtr->nextPtr = currentPtr;
        } /* end else */
    } /* end if */
    else {
        printf( "%c not inserted. No memory available.\n", value );
    } /* end else */
} /* end function insert */
 
int isEmpty( ListNodePtr sPtr )
{
    return sPtr == NULL;
}
 
/* Print the list */
void printList( ListNodePtr currentPtr )
{
    if ( currentPtr == NULL ) {
        printf( "List is empty.\n\n" );
    } 
    else {
        printf( "The list is:\n" );
        /* while not the end of the list */
        while ( currentPtr != NULL ) {
            printf( "%c --> ", currentPtr->data );
            currentPtr = currentPtr->nextPtr;
        } /* end while */
        printf( "NULL\n\n" );
    } /* end else */
} /* end function printList */
 
 
 ListNodePtr MergeLists(ListNodePtr list1, ListNodePtr list2) {
       ListNodePtr head;
  if (list1 == NULL) return list2;
  if (list2 == NULL) return list1;
 
 
  if (list1->data > list2->data) {
    head = list1;
  } else {
    head = list2;
    list2 = list1;
    list1 = head;
  }
  while(list1->nextPtr != NULL) {
      if (list1->nextPtr->data > list2->data) {
          ListNodePtr tmp = list1->nextPtr;
          list1->nextPtr = list2;
      list2 = tmp;
    }
      list1 = list1->nextPtr;
  } 
  list1->nextPtr = list2;
 
  return head;
}