【LeetCode】双链表——设计链表

链接：https://leetcode-cn.com/explore/learn/card/linked-list/196/doubly-linked-list/759/

代码：

class MyLinkedList {
struct ListNode{
    int val;
    ListNode *prev,*next;
    ListNode(int val):val(val),prev(NULL),next(NULL){}
};
private:
    ListNode *head;
    ListNode *tail;
    int size;
private:
    ListNode* getNode(int index)//获得目标节点，因为是双向链表，通过判断目标节点位置在前半段还是后半段来决定从head开始搜索还是从tail开始搜索
    {
        if(index<0||index>=size) return NULL;
        ListNode* p;
        if(size/2>index)
        {
            p=head;
            while(index>0)
            {
                p=p->next;
                index--;
            }          
        }
        else
        {
            p=tail;
            while(index<size-1)
            {
                p=p->prev;
                index++;
            }
        }
        return p;
        
    }
public:
    /** Initialize your data structure here. */
    MyLinkedList():head(NULL),tail(NULL),size(0) {}
    
    /** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
    int get(int index) {
        if(getNode(index)==NULL) return -1;
        else return getNode(index)->val;
    }
    
    /** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */
    void addAtHead(int val) {
        ListNode* node=new ListNode(val);
        size++;
        if(head==NULL)
        {
            head=node;
            tail=node;
        }
        else
        {
            node->next=head;
            head->prev=node;
            head=node;
        }
    }
    
    /** Append a node of value val to the last element of the linked list. */
    void addAtTail(int val) {
        ListNode* node=new ListNode(val);
        size++;
        if(tail==NULL)
        {
            head=node;
            tail=node;
        }
        else
        {
            tail->next=node;
            node->prev=tail;
            tail=node;
        }
    }
    
    /** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
    void addAtIndex(int index, int val) {
        if(index>size) return;
        if(index==size) 
        {
            addAtTail(val);
            return;
        }
        if(index<=0)
        {
            addAtHead(val);
            return;
        }
        ListNode* node=getNode(index);
        ListNode* n=new ListNode(val);
        n->next=node;
        n->prev=node->prev;
        node->prev->next=n;
        node->prev=n;
        size++;
    }
    
    /** Delete the index-th node in the linked list, if the index is valid. */
    void deleteAtIndex(int index) {
        ListNode* node=getNode(index);
        if(node)
        {
            if(node==head)
            {
                head=head->next;
                if(head) head->prev=NULL;
                node->next=NULL;
            }
            if(node==tail)
            {
                tail=tail->prev;
                if(tail) tail->next=NULL;
                node->prev=NULL;
            }
            if(node->next) node->prev->next=node->next;
            if(node->prev) node->next->prev=node->prev;
            delete node;
            size--;
        }
    }
};

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList* obj = new MyLinkedList();
 * int param_1 = obj->get(index);
 * obj->addAtHead(val);
 * obj->addAtTail(val);
 * obj->addAtIndex(index,val);
 * obj->deleteAtIndex(index);
 */

参考：https://leetcode-cn.com/problems/design-linked-list/solution/cxiang-xi-ti-jie-dai-tu-jie-by-youlookdeliciousc/