代码随想录算法训练营Day3 203.移除链表元素，707.设计链表，206.反转链表

目录

203.移除链表元素

707.设计链表

206.反转链表 

203.移除链表元素

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        //头结点值等于val，删除头结点,注意用的是while，因为我们删了一个头结点，但并不意味着下一个节点不等val(比如第2，第3个如果都等val)
        while(head != NULL && head->val == val){
            ListNode *tmp = head;//设一个临时节点指向头指针，便于删除
            head = head->next;//让头指针指向下一个节点，下一个节点变成头结点
            delete tmp;
        }
        //头结点值不等于val
        ListNode* p = head;//让我们创建的指针指向头指针
        while(p!= NULL && p->next != NULL){//循环直到下一个指针为空为止
            if(p->next->val == val){//如果等于了要删除的元素
                ListNode *tmp = p->next;//设一个临时变量，来记录值等于val的节点，然后释放他
                p->next = p->next->next;//跨过要删除的那个指针，然后接到下一个去，相当于这里其实涉及了三个节点 
                delete tmp;//删除该节点
            }else{
                p = p->next;
            }
        }
        return head;
    }
};

707.设计链表

typedef struct {
    int val;
    struct MyLinkedList* next;
} MyLinkedList;
MyLinkedList* myLinkedListCreate() {
    MyLinkedList* head = (MyLinkedList*)malloc(sizeof(MyLinkedList));
    head->next = NULL;
    return head;
}
int myLinkedListGet(MyLinkedList* obj, int index) {
    MyLinkedList* cur = obj->next;//obj是头指针，obj->next才是首节点
    for (int i = 0; cur != NULL; i++) {//因为index是下标所以不用-1之类的
        if (i == index) {
            return cur->val;
        } else {
            cur = cur->next;
        }
    }
    return -1;
}
void myLinkedListAddAtHead(MyLinkedList* obj, int val) {
    MyLinkedList* nhead = (MyLinkedList*)malloc(sizeof(MyLinkedList));
    nhead->val = val;
    nhead->next = obj->next;
    obj->next = nhead;
}
void myLinkedListAddAtTail(MyLinkedList* obj, int val) {
    MyLinkedList* cur = obj;
    while (cur->next != NULL) {
        cur = cur->next;
    }
    MyLinkedList* ntail = (MyLinkedList*)malloc(sizeof(MyLinkedList));
    ntail->val = val;
    ntail->next = NULL;
    cur->next = ntail;
}
void myLinkedListAddAtIndex(MyLinkedList* obj, int index, int val) {
    if (index == 0) {
        myLinkedListAddAtHead(obj, val);
        return;
    }
    MyLinkedList* cur = obj->next;
    for (int i = 1; cur != NULL; i++) {
        if (i == index) {
            MyLinkedList* newnode = (MyLinkedList*)malloc(sizeof(MyLinkedList));
            newnode->val = val;
            newnode->next = cur->next;
            cur->next = newnode;
            return;
        } else {
            cur = cur->next;
        }
    }
}
void myLinkedListDeleteAtIndex(MyLinkedList* obj, int index) {
    if (index == 0) {
        MyLinkedList* tmp = obj->next;
        if (tmp != NULL) {
            obj->next = tmp->next;
            free(tmp);
        }
        return;
    }
    MyLinkedList* cur = obj->next;
    for (int i = 1; cur != NULL && cur->next != NULL; i++) {
        if (i == index) {
            MyLinkedList* tmp = cur->next;
            if (tmp != NULL) {
                cur->next = tmp->next;
                free(tmp);
            }
            return;
        } else {
            cur = cur->next;
        }
    }
}
void myLinkedListFree(MyLinkedList* obj) {
    while (obj != NULL) {
        MyLinkedList* tmp = obj;
        obj = obj->next;
        free(tmp);
    }
}

206.反转链表 

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* fast = head;//从先去把这个链表遍历一遍到最后，取到最后的位置
        ListNode* slow = head;//指向头指针和快指针进行交换
        int tmp;//用来临时存储val值，交换用的
        int cnt = 0;//负责记录链表长度，然后算出移动次数
        int quanShu = 0;//圈数
        int num = 0;
        int n = 0;
        if(fast == nullptr) return head;
        while(fast != nullptr){//判断下一个数是不是空的
            fast = fast->next;
            cnt++;
        }
        fast = slow;
        quanShu = cnt / 2;//现在的cnt就是圈数了
        while((quanShu--) > 0){//交换数值
            for(int i = num; i next;
            }
                //快指针移动完之后，开始交换值
                tmp = fast->val;
                fast->val = slow->val;
                slow->val = tmp;
                slow = slow->next;//慢指针前移
                fast = slow;//重置快指针的位置
                cnt--;
                num++;
        }
        return head;
    }
};