day03 - 链表part01

203. 移除链表元素

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

        //虚拟头结点
        ListNode* tmp_head = new ListNode(0);
        tmp_head->next = head;
        ListNode* cur = tmp_head;
        while(cur != NULL){
            if(cur->next != nullptr && cur->next->val == val){
                ListNode* tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            }else{
                cur = cur->next;
            }
        }
        head = tmp_head->next;
        delete tmp_head;
        return head;
    }
};

 

707. 设计链表

class MyLinkedList {

public:
    struct my_list{
        int val;
        my_list* next;
        my_list(int val):val(val),next(nullptr){}
    };

    MyLinkedList() {
        dummy_list = new my_list(0);
        _size = 0;
    }

    void log(){
        return;
        my_list* tmp = dummy_list;
        while(tmp != nullptr){
            cout << tmp->val << "," ;
            tmp = tmp->next;
        }
        cout << endl;
    }
    
    int get(int index) {
        //log();
        if(_size == 0 || index < 0 || index >= _size)
            return -1;
        my_list* tmp = dummy_list->next;
        while(index --)
            tmp = tmp->next;
        return tmp->val;
    }
    
    void addAtHead(int val) {
        my_list* tmp = new my_list(val);
        tmp->next = dummy_list->next;
        dummy_list->next = tmp;
        _size++;
        log();
    }
    
    void addAtTail(int val) {
        my_list* tmp = dummy_list;
        while(tmp->next != NULL)
            tmp = tmp->next;
        my_list* tmp2 = new my_list(val);
        tmp->next = tmp2;
        _size++;
        log();
    }
    
    void addAtIndex(int index, int val) {
        //cout << "size:" << _size << endl;
        if(index > _size)
            return;
        my_list* tmp = dummy_list;
        while(--index >= 0)
            tmp = tmp->next;
        //cout << "index:" << index << endl;
        my_list* tmp2 = new my_list(val);
        
        if(tmp->next != NULL)
            tmp2->next = tmp->next;
        tmp->next = tmp2;
        _size++;
        log();
    }
    
    void deleteAtIndex(int index) {
        if(index >= _size)
            return;
        my_list* tmp = dummy_list;
        while(--index >= 0)
            tmp = tmp->next;
        if(index < _size - 1){
            my_list* tmp2 = tmp->next;
            tmp->next = tmp->next->next;
            delete tmp2;
        }else{
            tmp->next = tmp->next->next;
        }
        _size--;
        log();
    }

private:
    my_list* dummy_list;
    int _size;
};

/*
class MyLinkedList {
public:
    // 定义链表节点结构体
    struct LinkedNode {
        int val;
        LinkedNode* next;
        LinkedNode(int val):val(val), next(nullptr){}
    };

    // 初始化链表
    MyLinkedList() {
        _dummyHead = new LinkedNode(0); // 这里定义的头结点 是一个虚拟头结点，而不是真正的链表头结点
        _size = 0;
    }

    // 获取到第index个节点数值，如果index是非法数值直接返回-1， 注意index是从0开始的，第0个节点就是头结点
    int get(int index) {
        if (index > (_size - 1) || index < 0) {
            return -1;
        }
        LinkedNode* cur = _dummyHead->next;
        while(index--){ // 如果--index 就会陷入死循环
            cur = cur->next;
        }
        return cur->val;
    }

    // 在链表最前面插入一个节点，插入完成后，新插入的节点为链表的新的头结点
    void addAtHead(int val) {
        LinkedNode* newNode = new LinkedNode(val);
        newNode->next = _dummyHead->next;
        _dummyHead->next = newNode;
        _size++;
    }

    // 在链表最后面添加一个节点
    void addAtTail(int val) {
        LinkedNode* newNode = new LinkedNode(val);
        LinkedNode* cur = _dummyHead;
        while(cur->next != nullptr){
            cur = cur->next;
        }
        cur->next = newNode;
        _size++;
    }

    // 在第index个节点之前插入一个新节点，例如index为0，那么新插入的节点为链表的新头节点。
    // 如果index 等于链表的长度，则说明是新插入的节点为链表的尾结点
    // 如果index大于链表的长度，则返回空
    // 如果index小于0，则在头部插入节点
    void addAtIndex(int index, int val) {

        if(index > _size) return;
        if(index < 0) index = 0;        
        LinkedNode* newNode = new LinkedNode(val);
        LinkedNode* cur = _dummyHead;
        while(index--) {
            cur = cur->next;
        }
        newNode->next = cur->next;
        cur->next = newNode;
        _size++;
    }

    // 删除第index个节点，如果index 大于等于链表的长度，直接return，注意index是从0开始的
    void deleteAtIndex(int index) {
        if (index >= _size || index < 0) {
            return;
        }
        LinkedNode* cur = _dummyHead;
        while(index--) {
            cur = cur ->next;
        }
        LinkedNode* tmp = cur->next;
        cur->next = cur->next->next;
        delete tmp;
        //delete命令指示释放了tmp指针原本所指的那部分内存，
        //被delete后的指针tmp的值（地址）并非就是NULL，而是随机值。也就是被delete后，
        //如果不再加上一句tmp=nullptr,tmp会成为乱指的野指针
        //如果之后的程序不小心使用了tmp，会指向难以预想的内存空间
        tmp=nullptr;
        _size--;
    }

    // 打印链表
    void printLinkedList() {
        LinkedNode* cur = _dummyHead;
        while (cur->next != nullptr) {
            cout << cur->next->val << " ";
            cur = cur->next;
        }
        cout << endl;
    }
private:
    int _size;
    LinkedNode* _dummyHead;

};
*/


/**
 * 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

/**
 * 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* reverseList(ListNode* head) {
        ListNode* tmp;
        ListNode* pre = NULL;
        ListNode* cur = head;

        while(cur){
            tmp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
};