[英]Why isn't my reverse(); function working?
我正在 C 中編寫一個用於反轉循環單鏈表的程序。 由於某種原因,我不斷收到分段錯誤。 我確定問題出在reverse
function 上,因為我嘗試評論 function 調用,程序運行良好。
對於我的reverse()
function,我使用了 3 個指針: prev
、 next
和curr
。 邏輯是我將運行一個循環,直到curr
獲取head
的地址,該地址將存儲在最后一個節點的link
部分,因為它是一個循環鏈表。 我將使用prev
指針繼續更新curr->link
,這會將其鏈接從下一個節點更改為其前一個節點。
當循環中斷時, head->link = prev;
和head = prev;
將更新各自的地址,使它們指向反向列表的第一個節點。
//reversing CLL
#include <stdio.h>
#include <stdlib.h>
struct node {
int data;
struct node *link;
} *head;
void reverse() {
struct node *prev = NULL, *curr = head, *next;
while (curr != head) {
next = curr->link;
curr->link = prev;
prev = curr;
curr = next;
}
head->link = prev;
head = prev;
}
void createList(int n) {
int i, data;
head = (struct node *)malloc(sizeof(struct node));
struct node *ptr = head, *temp;
printf("Enter data of node 1\t");
scanf("%d", &data);
head->data = data;
head->link = NULL;
for (i = 2; i <= n; i++) {
temp = (struct node *)malloc(sizeof(struct node));
printf("Enter data of node %d\t", i);
scanf("%d", &data);
temp->data = data;
temp->link = NULL;
ptr->link = temp;
ptr = ptr->link;
}
ptr->link = head;
}
void disp() {
struct node *ptr = head;
do {
printf("%d\t", ptr->data); //gdb debugger shows problem is in this line
ptr = ptr->link;
} while (ptr != head);
}
int main() {
int n;
printf("Enter no of nodes to be created\t");
scanf("%d", &n);
createList(n);
printf("\n\nList is displayed below!\n");
disp();
printf("\n\nReversing list ...\n");
reverse(); // on commenting this call, disp() function
// works accurately showing node data non-reversed
disp();
printf("\n\nList successfully reversed!\n");
}
reverse()
function 中的循環立即退出,因為curr
已初始化為head
的值,因此測試while (curr != head)
在第一次迭代時為假。
reverse()
then sets head->link
to NULL
and finally head
is also set to NULL
(the initial value of prev
), which explains the segmentation fault in the subsequent disp()
function where you use a do { } while (pre != head)
無法處理空列表。
這是修改后的版本:
#include <stdio.h>
#include <stdlib.h>
struct node {
int data;
struct node *link;
};
struct node *reverse(struct node *head) {
struct node *prev = NULL, *curr = head;
if (head) {
do {
struct node *next = curr->link;
curr->link = prev;
prev = curr;
curr = next;
} while (curr != head);
curr->link = prev;
head = prev;
}
return head;
}
struct node *createList(int n) {
struct node *head = NULL, *tail = NULL, *temp;
int i;
for (i = 1; i <= n; i++) {
temp = (struct node *)malloc(sizeof(struct node));
temp->data = 0;
temp->link = NULL;
printf("Enter data of node %d\t", i);
scanf("%d", &temp->data);
if (head == NULL) {
head = temp;
} else {
tail->link = temp;
}
tail = temp;
temp->link = head;
}
return head;
}
void disp(struct node *head) {
if (head) {
struct node *ptr = head;
do {
printf("%d\t", ptr->data);
ptr = ptr->link;
} while (ptr != head);
}
}
int main() {
struct node *head;
int n = 0;
printf("Enter no of nodes to be created\t");
scanf("%d", &n);
head = createList(n);
printf("\n\nList is displayed below!\n");
disp(head);
printf("\n\nReversing list ...\n");
head = reverse(head);
disp(head);
printf("\n\nList successfully reversed!\n");
// should free the list
return 0;
}
對於初學者來說,使用全局變量head
是個壞主意
struct node {
int data;
struct node *link;
} *head;
在這種情況下,函數依賴於全局變量,您不能在程序中使用多個列表。
由於這個初始化
struct node *prev = NULL, *curr = head, *next;
^^^^^^^^^^^^
while 循環的條件
while (curr != head) {
is 永遠不會計算為 true,因為最初指針curr
等於指針head
。
此外,如果列表為空,則此語句
head->link = prev;
調用未定義的行為。
這是一個演示程序,顯示了如何在 main 中聲明列表然后反轉。
#include <stdio.h>
#include <stdlib.h>
struct node
{
int data;
struct node *link;
};
size_t assign( struct node **head, const int a[], size_t n )
{
while ( *head )
{
struct node *tmp = *head;
*head = ( *head )->link;
free( tmp );
}
size_t total = 0;
struct node *first = NULL;
while ( total < n && ( *head = malloc( sizeof( struct node ) ) ) != NULL )
{
( *head )->data = a[total];
( *head )->link = NULL;
++total;
if ( first == NULL ) first = *head;
head = &( *head )->link;
}
if ( first != NULL ) *head = first;
return total;
}
void display( const struct node *head )
{
if ( head != NULL )
{
const struct node *current = head;
do
{
printf( "%d -> ", current->data );
} while ( ( current = current->link ) != head );
}
puts( "null" );
}
struct node * reverse( struct node **head )
{
if ( *head )
{
struct node *last = *head;
struct node *prev = NULL;
while ( ( *head )->link != last )
{
struct node *current = *head;
*head = ( *head )->link;
current->link = prev;
prev = current;
}
( *head )->link = prev;
last->link = *head;
}
return *head;
}
int main(void)
{
struct node *head = NULL;
int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
assign( &head, a, sizeof( a ) / sizeof( *a ) );
display( head );
display( reverse( &head ) );
display( reverse( &head ) );
return 0;
}
程序 output 是
0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> null
9 -> 8 -> 7 -> 6 -> 5 -> 4 -> 3 -> 2 -> 1 -> 0 -> null
0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> null
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