[英]Why are we using pointer to pointer for one and a normal pointer to another?
// A complete working C++ program to demonstrate
// all insertion methods on Linked List
#include <bits/stdc++.h>
using namespace std;
// A linked list node
class Node
{
public:
int data;
Node *next;
};
/* Given a reference (pointer to pointer)
to the head of a list and an int, inserts
a new node on the front of the list. */
void push(Node** head_ref, int new_data)
{
/* 1. allocate node */
Node* new_node = new Node();
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head */
new_node->next = (*head_ref);
/* 4. move the head to point to the new node */
(*head_ref) = new_node;
}
/* Given a node prev_node, insert a new node after the given
prev_node */
void insertAfter(Node* prev_node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_node == NULL)
{
cout<<"The given previous node cannot be NULL";
return;
}
/* 2. allocate new node */
Node* new_node = new Node();
/* 3. put in the data */
new_node->data = new_data;
/* 4. Make next of new node as next of prev_node */
new_node->next = prev_node->next;
/* 5. move the next of prev_node as new_node */
prev_node->next = new_node;
}
/* Given a reference (pointer to pointer) to the head
of a list and an int, appends a new node at the end */
void append(Node** head_ref, int new_data)
{
/* 1. allocate node */
Node* new_node = new Node();
Node *last = *head_ref; /* used in step 5*/
/* 2. put in the data */
new_node->data = new_data;
/* 3. This new node is going to be
the last node, so make next of
it as NULL*/
new_node->next = NULL;
/* 4. If the Linked List is empty,
then make the new node as head */
if (*head_ref == NULL)
{
*head_ref = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last->next != NULL)
{
last = last->next;
}
/* 6. Change the next of last node */
last->next = new_node;
return;
}
// This function prints contents of
// linked list starting from head
void printList(Node *node)
{
while (node != NULL)
{
cout<<" "<<node->data;
node = node->next;
}
}
/* Driver code*/
int main()
{
/* Start with the empty list */
Node* head = NULL;
// Insert 6. So linked list becomes 6->NULL
append(&head, 6);
// Insert 7 at the beginning.
// So linked list becomes 7->6->NULL
push(&head, 7);
// Insert 1 at the beginning.
// So linked list becomes 1->7->6->NULL
push(&head, 1);
// Insert 4 at the end. So
// linked list becomes 1->7->6->4->NULL
append(&head, 4);
// Insert 8, after 7. So linked
// list becomes 1->7->8->6->4->NULL
insertAfter(head->next, 8);
cout<<"Created Linked list is: ";
printList(head);
return 0;
}
// from geeksforgeek
Why the append and push function have a pointer to pointer (head_ref), but the insertAfter function has a normal pointer (prev_node)?为什么 append 和 push 函数有一个指向指针的指针(head_ref),而 insertAfter 函数有一个普通的指针(prev_node)? As we are changing them whether it is the head or the previous node, so it should be same approach(types of pointer).
由于我们正在更改它们是头节点还是前一个节点,因此应该是相同的方法(指针类型)。
Can I do use a normal pointer instead of a pointer to pointer for the same?我可以使用普通指针而不是指向指针的指针吗?
The functions push
and append
both need to be able to change the pointer to the head node.函数
push
和append
都需要能够改变指向头节点的指针。
If they were only passed a Node*
pointing to the head, then they would only be able to change their local copy of that pointer to the head node.如果他们只传递了一个指向头
Node*
,那么他们将只能将该指针的本地副本更改为头节点。 However, they need to be able to change the original pointer to the head node, which is the variable head
in the function main
.但是,它们需要能够更改指向头节点的原始指针,即函数
main
的变量head
。 That is why they need a pointer to the variable head
in the function main
, so both functions need to be passed a Node**
.这就是为什么他们需要一个指向函数
main
变量head
的指针,所以两个函数都需要传递一个Node**
。
In contrast to the functions push
and append
, the function insertAfter
will never need to change the pointer that is being passed to the function.与
push
和append
函数insertAfter
,函数insertAfter
永远不需要更改传递给函数的指针。 That is why it is sufficient to pass a Node*
.这就是为什么传递一个
Node*
就足够了。
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