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#include <stdlib.h>
struct listNode { // self-referential structure
char info;
struct listNode *nextPtr;
};
typedef struct listNode LISTNODE;
typedef LISTNODE *LISTNODEPTR;
void insert( LISTNODEPTR *, char );
char delete( LISTNODEPTR *, char );
int isEmpty( LISTNODEPTR );
void printList( LISTNODEPTR );
void instructions( void );

int main()
{
    LISTNODEPTR startPtr = NULL;
    int choice;
    char item;
    instructions(); // display the menu
    scanf( "%d", &choice );
    while ( choice != 0 ) {
        switch (choice) {
            case 1:
                printf( "Enter a character: " );
                scanf( " %c", &item );
                insert( &startPtr, item );
                printList( startPtr );
                break;
            case 2:
                if ( !isEmpty( startPtr ) ) {
                    printf( "Enter character to be deleted: " );
                    scanf( " %c", &item );
                    if ( delete( &startPtr, item ) ) {
                        printf( "%c deleted.\n", item );
                        printList( startPtr );
                    }
                    else
                        printf( "%c not found.\n\n", item );
                }
                else
                    printf( "List is empty.\n\n" );
                    break;
            default:
                printf( "Invalid choice.\n\n" );
                break;
        }
        instructions(); // display the menu
        scanf( "%d", &choice );
    }
    printf( "Program is going to exit. Press any key to continue\n" );
    getch();
    return 0;
}

// Print the instructions
void instructions( void )
{
    printf( "Enter your choice:\n"
    " 1 to insert an element into the list.\n"
    " 2 to delete an element from the list.\n"
    " 0 to end.\n" );
    printf( " ? " );
}

// Insert a new item into the list in sorted order
void insert( LISTNODEPTR *sPtr, char item )
{
    LISTNODEPTR newNodeLink, prevNodeLink, curNodeLink;
    newNodeLink =(LISTNODEPTR) malloc( sizeof( LISTNODE ) );
    if ( newNodeLink != NULL ) { // is space available
        newNodeLink->info = item;
        newNodeLink->nextPtr = NULL;

        prevNodeLink = NULL;
        curNodeLink = *sPtr;
        while ( curNodeLink != NULL && item > curNodeLink->info ) {
            prevNodeLink = curNodeLink; // walk to ...
            curNodeLink = curNodeLink->nextPtr; // ... next node
        }
        if ( prevNodeLink == NULL ) {
            newNodeLink->nextPtr = *sPtr;
            *sPtr = newNodeLink;
        }
        else {
            prevNodeLink->nextPtr = newNodeLink;
            newNodeLink->nextPtr = curNodeLink;
        }
    }
    else
        printf( "%c not inserted. No memory available.\n", item );
}

// Delete a list element
char delete( LISTNODEPTR *sPtr, char item )
{
    LISTNODEPTR prevNodeLink, curNodeLink, tempPtr;
    if ( item == ( *sPtr )->info ) {
        tempPtr = *sPtr;
        *sPtr = ( *sPtr )->nextPtr; // de-thread the node
        free( tempPtr ); // free the de-threaded node
        return item;
    }
    else {
        prevNodeLink = *sPtr;
        curNodeLink = ( *sPtr )->nextPtr;
        while ( curNodeLink != NULL && curNodeLink->info != item ) {
            prevNodeLink = curNodeLink; // walk to ...
            curNodeLink = curNodeLink->nextPtr; // ... next node
        }
        if ( curNodeLink != NULL ) {
            tempPtr = curNodeLink;
            prevNodeLink->nextPtr = curNodeLink->nextPtr;
            free( tempPtr );
            return item;
        }
    }
    return '\0';
}

// Return 1 if the list is empty, 0 otherwise
int isEmpty( LISTNODEPTR sPtr )
{
    return sPtr == NULL;
}

// Print the list
void printList( LISTNODEPTR curNodeLink )
{
    if ( curNodeLink == NULL )
        printf( "List is empty.\n\n" );
    else {
        printf( "The list is:\n" );
        while ( curNodeLink != NULL ) {
            printf( "%c --> ", curNodeLink->info );
            curNodeLink = curNodeLink->nextPtr;
        }
        printf( "NULL\n\n" );
        system("pause");
    }
}