如何理解单链列表的创建

Question

Problem

I have some difficulties in understanding the code (not the meaning) of a tutorial to create a singly linked list with c++.

Code

typedef int ElemType;   
struct Node{
    ElemType data;
    Node *next;
};

class LinkList{
private:
    Node *Head;
public:
    LinkList();         
    ~LinkList();
    void CreateList1(int n);
    void CreateList2(int n);
    void ListInsert(int i, int e);
    int ListDelete(int i);
    int GetElem(int i);
    int LocateElem(int e);
    int ListLength();
};

void LinkList::CreateList1(int n) {
    //create a linked list by inserting the element in the head
    Node *p, *s;
    p = Head;
    cout<<"请依次输入"<<n<<"个数据元素值"<<endl;
    for (int i =1; i<n; i++){
        s = new Node;           
        cin>>s->data;
        s->next=p->next;       
        p->next=s;              // what does it mean? I don't get it.
    }
}

void LinkList::CreateList2(int n) {
    //create a linked list by inserting the element in the end
    Node *p, *s;
    p = Head;
    cout << "请依次输入" << n << "个数据元素值" << endl;
    for (int i = 1; i < n; i++) {
        s = new Node;          
        cin >> s->data;
        p->next=s;              
        p=s;                   // what does it mean? I don't get it.
    }
}

Note

The code snippet that I do not understand is commented. Anyone can explain the code with instructive words or figures? Thanks in advance.

Answer 1

This: [ ] is a node and: -> is used to show where the node points to.

1st case:

• HEAD will point to the new node
• the new node will point where HEAD was previously pointing

 p->next=s; // what does it mean? I don't get it 

It means: HEAD should now point to the new node

---

[HEAD]->

iteration 1:
-------------------------------
s = new Node      : [S1]
s->next = p->next : [S1]->
p->next = s       : [HEAD]->[S1]

iteration 2:
-------------------------------
s = new Node      : [S2]
s->next = p->next : [S2]->[S1]
p->next = s       : [HEAD]->[S2]->[S1]

iteration 3:
-------------------------------
s = new Node      : [S3]
s->next = p->next : [S3]->[S2]
p->next = s       : [HEAD]->[S3]->[S2]->[S1]

2nd case:

• HEAD will point to the new node
• The new node becomes HEAD

 p=s; // what does it mean? I don't get it 

It means: the new node now also becomes HEAD

---

[HEAD]->

iteration 1:
-------------------------------
s = new Node      : [S1]
p->next = s       : [HEAD]->[S1]
p = s             : [HEAD,S1]               // S1 is the HEAD now

iteration 2:
-------------------------------
s = new Node      : [S2]
p->next = s       : [HEAD,S1]->[S2]
p = s             : [S1]->[HEAD,S2]        // S2 is the HEAD now

iteration 3:
-------------------------------
s = new Node      : [S3]
p->next = s       : [HEAD,S2]->[S3]
p = s             : [S1]->[S2]->[HEAD,S3]  // S3 is the HEAD now

Answer 2

p is the pointer to the insertion point.

The new node is inserted AFTER p .

In the first case, p->next=s , we hang the new node s off the node pointed to by p , but p itself does not change. The next node will still be inserted right after Head .

In the second case, p->next=s is still done, but then we do p=s , so the insertion point p moves to the last element of the list, s . The next node will be inserted at the end of the list, not at the head.