刷题记录|Day3 ● 203.移除链表元素 ● 707.设计链表 ● 206.反转链表

203.移除链表元素

题目描述

解决方法

方法1 直接在原始链表上进行操作

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        
        while (head != NULL && head->val == val) { 
            ListNode* tmp = head;
            head = head->next;
            delete tmp;
        }

        ListNode* cur = head;
        while (cur != NULL && cur->next!= NULL) {
            if (cur->next->val == val) {
                ListNode* tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            } else {
                cur = cur->next;
            }
        }
        return head;
    }
};

707.设计链表

题目描述

解决方法

class MyLinkedList {
public:
    struct LinkedNode{
        int val;
        LinkedNode *next;
        LinkedNode(int val)
            :val(val),next(nullptr)
            {}
    };
    MyLinkedList() {
        dumyHead = new LinkedNode(0);
        size = 0;

    }
    
    int get(int index) {
        if (index > (size - 1) || index < 0) {
            return -1;
        }
        LinkedNode* cur = dumyHead->next;
        while(index--){ // 如果--index 就会陷入死循环
            cur = cur->next;
        }
        return cur->val;

    }
    
    void addAtHead(int val) {
        
        LinkedNode * newNode = new LinkedNode(val);
        newNode->next = dumyHead->next;
        dumyHead->next = newNode;
        size++;
    }
    
    void addAtTail(int val) {
        LinkedNode *Node = new LinkedNode(val);
        LinkedNode* cur = dumyHead;
        while(cur->next)
        {
            cur = cur->next;
        }
        cur->next = Node;
        size++;
    }
    
    void addAtIndex(int index, int val) {
        if(index>size)return;
        if(index<0) index = 0;
        LinkedNode *Node = new LinkedNode(val);
        LinkedNode *cur = dumyHead;
        while(index--)
        {
            cur = cur->next;
        }
        LinkedNode *temp = cur->next;
        cur->next = Node;
        Node->next = temp;
        size++;

    }
    
    void deleteAtIndex(int index) {
        if(index>=(size)||index<0) return;
        LinkedNode *cur = dumyHead;
        while(index--)
        {
            cur = cur->next;
        }
        LinkedNode *tmp = cur->next;
        cur->next = cur->next->next;
        delete tmp;
        size--; 



    }
private:
    int size;
    LinkedNode *dumyHead;
};

/**
 * 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);
 */

206.反转链表

题目描述

解题思路

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == nullptr) return head;
                
        ListNode *p = head->next;
        head->next  = nullptr;
        ListNode *q = nullptr;
         
        while(p!=nullptr)
        {
            q = p->next;
            p->next = head;
            head = p;
            p = q;
            

        }
        return head;

    }
};