如何刪除列表中的最後一個元素用C

Question

我在Ç使用一種自我指涉的結構創建的列表，我想通了如何做好每一件事可能的列表中，但刪除最後一個元素，這是令人費解我現在幾個星期了，請幫我 這就是我所做的。 爲了得到這個程序的概念，你只需要看功能main，和Delete，也許insert。請注意，這個代碼是在Ubuntu上使用gcc編譯的。

/* self-referencial structure */ 
struct ListNode 
{ 
    char data; 
struct ListNode *nextNode ; 
}; 


typedef struct ListNode ListNode; 
typedef ListNode *ListNodePtr ; 

/* function protypes */ 
void insert(ListNodePtr *sptr, char value); // insert element in list in alphabet order 
void Delete (ListNodePtr *sptr);// PROBLEM HERE; remove last element from list 
void printList(ListNodePtr);// output the contents of the list 
void instructions(); // print instruction on screen 

int main(void) 
{ 
ListNodePtr startPtr = NULL ; // there are initially no nodes 
int choice; 
char item ;// char to be placed in list 

/* Give instruction s and allow user to make a choice 
* insert , print, delete. 
*/ 
instructions(); 
printf("?"); 
scanf("%d", &choice); 

/* loop until sentinel value 3 */ 
while (!(choice == 4)) 
{ 
    switch(choice) 
    { 
    case 1: 
     printf("Enter a character"); 
     scanf("\n%c",&item); 
     insert(&startPtr, item); 
     printList(startPtr); 
     break ; 

    case 2: 
     /* If the list is not empty */ 
     Delete(&startPtr); 
     printList(startPtr); 
     // if it is printf("the list is empty"); 
     break; 
    case 3: 
     printList(startPtr); 
     break; 
    default: 
     printf("Invalid choice"); 
     break; 
    }// end switch 
    printf("?"); 
    scanf("%d",&choice); 
}// end while 
return 0 ; 
}// END FUNCTION MAIN 

/* display program choices/instruction to user */ 
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" 
    "3 to print the list\n" 
    "4 to end.\n"); 
} 

void insert(ListNodePtr *sptr, char value) 
{ 
ListNodePtr currentPtr; 
ListNodePtr previousPtr; 
ListNodePtr newNode ; 


/* create an area in memory the size of ListNode and 
*allocate to newPtr */ 
newNode = malloc(sizeof(ListNode)); // create new node 

// if space is available 
if (newNode != NULL) 
{ 
    newNode->data = value; 
    newNode->nextNode = NULL; 

    previousPtr = NULL ; 
    currentPtr = *sptr ; 

    /* walk to correct location in the list */ 
    while (currentPtr != NULL && value > currentPtr->data) 
    { 
     previousPtr = currentPtr; 
     currentPtr = currentPtr->nextNode; 
    }// end while 

    /* insert node at beginning of the list */ 
    if(previousPtr == NULL) 
    { 
     newNode->nextNode = *sptr; 
     *sptr = newNode; 
    }// end if 

    else 
    { 
     previousPtr->nextNode = newNode; 
     newNode->nextNode = currentPtr ; 
    } 
}// end if 
else 
{ 
    printf("%c not inserted , memory not available", value); 
} 

} 

void Delete(ListNodePtr *sptr) 
{ 
ListNodePtr previousPtr; 
ListNodePtr currentPtr; 
ListNodePtr tempPtr; 

// if there is only one element 
if((*sptr)->nextNode == NULL) 
{ 
    tempPtr = *sptr; // hold on to Node being removed 
    *sptr = (*sptr)->nextNode; 
    free(tempPtr); 
} 
else 
{ 
    previousPtr = *sptr ; 
    currentPtr = (*sptr)->nextNode; 

    // walk to end of the list 
    while(currentPtr->nextNode != NULL) 
    { 
     previousPtr = *sptr; 
     currentPtr = currentPtr->nextNode; 
    } 

    if(currentPtr != NULL) 
    { 
     // WHAT DO I DO HERE ??!!?? 
     // if the list has more than one two elements, 
     // all but the first element is deleted 

     previousPtr->nextNode = currentPtr->nextNode; 
     currentPtr = NULL ; 
     free(currentPtr); 
    }// end if 
}// end else 


}// END FUNCTION DELETE 

void printList(ListNodePtr currentPtr) 
{ 
if(currentPtr == NULL) 
printf("List is empty"); 
else 
{ 
    while(currentPtr != NULL) 
    { 
     printf("%c --> ", currentPtr->data); 
     currentPtr = currentPtr->nextNode; 
    } /* end while */ 
     printf("NULL\n\n"); 
    /* end else */ 


} 
} 

Answer 1

可能是這有助於

void Delete(ListNodePtr *sptr) 
{ 
ListNodePtr previousPtr; 
ListNodePtr currentPtr; 
ListNodePtr tempPtr; 

if((*sptr)->nextNode == NULL) 
{ 
    tempPtr = *sptr; // hold on to Node being removed 
    *sptr = (*sptr)->nextNode; 
    free(tempPtr); 
} 
else 
{ 
previousPtr = *sptr ; 
currentPtr = (*sptr)->nextNode; 

// walk to end of the list 
while(currentPtr->nextNode != NULL) 
{ 
    previousPtr=previousPtr->nextNode;//this always points to the previous node 
    currentPtr = currentPtr->nextNode; 
} 

//after loop ends currentPtr points to the last node 

if(currentPtr != NULL) 
{ 
    previousPtr->nextNode=NULL;//This deletes the currentPtr from the list 
    free(currentPtr); 
}// end if 
}// end else 
}// END FUNCTION DELETE