203.移除链表元素

https://leetcode.cn/problems/remove-linked-list-elements/description/

/**
 * 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) {
        //先删头节点
        while(head != nullptr && head->val == val)
        head = head->next;

        ListNode* cur = head;
        while(cur != nullptr && cur->next != nullptr)
        {
            if(cur->next->val == val)
            cur->next = cur->next->next;

            else                //这里else是为了保证，比如1 2 2 1，删完第一个2，如果直接后移cur，就不会判断第二个2了
            cur = cur->next;
        }
        return head;
    }
};

707. 设计链表

https://leetcode.cn/problems/design-linked-list/

class MyLinkedList {
public:
    struct Node {
        int val;
        Node *next;
        Node() : val(0), next(nullptr) {}
        Node(int x) : val(x), next(nullptr) {}
        Node(int x, Node *next) : val(x), next(next) {}
    };
    Node *dummyhead;
    int size;                       //size表示链表一共有几个元素，所以最大索引是size-1
    MyLinkedList() {
        dummyhead = new Node();
        size = 0;
    }
    int get(int index) {
        if(index < 0 || index >= size)      //保证一定可以获取到正确的索引位置的val
            return -1;
        Node* cur = dummyhead->next;        //cur从头节点开始，遍历index次
        while(index--)
            cur = cur->next;
        return cur->val;
    }
    void addAtHead(int val) {
        Node* node = new Node(val);
        node->next = dummyhead->next;
        dummyhead->next = node; 
        size++;
    }
    
    void addAtTail(int val) {
        Node* node = new Node(val);
        Node* cur = dummyhead;
        while(cur->next != nullptr)         //跑到尾，现在的cur就是最后一个节点
            cur = cur->next;
        cur->next = node;
        size++;
    }
    void addAtIndex(int index, int val) {
        if(index > size)                    //为什么没等号，比如一共三节点，在索引3前加就是尾插的意思，至于负数统统看成头插
            return;
        Node* node = new Node(val);
        Node* cur = dummyhead;              //需要锁定index索引的前一个，因为要求插入第index个的前一个
        while(index--) 
            cur = cur->next;

        node->next = cur->next;
        cur->next = node;
        size++;
    }
    void deleteAtIndex(int index) {
        if(index < 0 || index >= size)      //保证一定可以找到正确的索引位置的val
            return;
        Node* cur = dummyhead;              //保证找到index-1索引处的值

        while(index--) 
            cur = cur->next;

        Node* del = cur->next;
        cur->next = del->next;
        delete del;
        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);
 */

206.翻转链表

https://leetcode.cn/problems/reverse-linked-list/description/

/**
 * 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) {
        if(head == nullptr || head->next == nullptr) 
            return head;

        //思路 用temp记录当前的后面所有节点，即temp->next = remain，现在temp和cur都指向了remain，
        //就可以断掉cur->next，让cur指向它前面的那个，也就是cur->next = pre; 接下来，temp cur pre都后移一个，继续操作。
        ListNode* pre = nullptr;
        ListNode* cur = head;
        //ListNode* temp = cur;
        ListNode* temp = cur->next;
        while(cur != nullptr)
        {
            // cur->next = pre;        //这一步实际上改变了原链表的结构，所以temp->next也找不到了！！！！
            // pre = cur;
            // cur = temp->next;
            // temp = cur;
            cur->next = pre;        
            pre = cur;
            cur = temp;
            if(temp != nullptr)
            temp = cur->next;

        }
        return pre;
    }
};

19.删除链表的倒数第N个节点

https://leetcode.cn/problems/remove-nth-node-from-end-of-list/

/**
 * 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* removeNthFromEnd(ListNode* head, int n) {
        //好妙的双指针，移动快指针直至快慢差n，一起移动，快指针到结尾了，说明该删除慢指针了
        //这里需要推算一下，当fast是倒一了，比如要删除倒3，实际上他们之间只隔了1！但是为了方便删除，需要找到倒4操作！！！
        ListNode* dummyhead = new ListNode();
        dummyhead->next = head;
        ListNode* slow = dummyhead;
        ListNode* fast = dummyhead;
        
        //令fast等于第n-1索引的位置
        //题目已经说了n的范围合法！！不用判断!!
        for(int i = 0; i < n; i++)
            fast = fast->next;
        
        //现在slow和fast隔了n-1个，只要fast移到最后，slow的位置就是要删除的前一个，让这个的next跳过下一个即可
        while(fast->next!=nullptr)
        {
            fast = fast->next;
            slow = slow->next;
        }
        ListNode* del = slow->next;
        slow->next = slow->next->next;
        delete del;

        return dummyhead->next;
    }
};

24.两两交换链表中的节点

https://leetcode.cn/problems/swap-nodes-in-pairs/description/

/**
 * 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* swapPairs(ListNode* head) {
        if(head == nullptr || head->next == nullptr)
        return head;

        ListNode* cur = new ListNode();
        cur->next = head;
        ListNode* res = cur;
        while(cur->next!=nullptr && cur->next->next!=nullptr)
        {
            ListNode* tmp1 = cur->next;
            ListNode* tmp2 = cur->next->next;

            // cur->next = tmp2;
            // tmp2->next = tmp1;    
            //这样写会导致死循环。第一行表示剔除tmp1，第二行表示tmp2指向tmp1，而tmp1又指向tmp2，它应该指向tmp2下一个

            cur->next = tmp2;
            tmp1->next = tmp2->next;
            tmp2->next = tmp1;

            cur = cur->next->next; //实际上就是tmp2->next，也是tmp1->next
        }
        return res->next;
    }
};

链表相交

https://leetcode.cn/problems/intersection-of-two-linked-lists-lcci/description/

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        //k神的解法，A全部+B公共以前 = B全部+A公共以前
        ListNode* A = headA;
        ListNode* B = headB;
        while(A!=B)
        {
            //相当于if(A!=nullptr)   A = A->next； else A = headB
            A = A != nullptr? A->next : headB;
            B = B != nullptr? B->next : headA;
        }
        return A;
    }
};

142.环形链表

https://leetcode.cn/problems/linked-list-cycle-ii/description/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        //fast一次走两步，slow一次走一步，他们在环内相遇，如果到末尾还没相遇就是没有环，
        // fast - slow = nb， fast = 2*slow ==> slow = nb, fast = 2nb！！！
        // 一步一步地走，必然可以停留在入口处，slow = nb  想办法让他再走a步，就可以找到入口！
        // 相遇后，fast回到起点，一次走一步走a步，slow跟着走a步，二者再次相恰好是环入口

        /*重点：① 二者第一次相遇 slow = nb
                ②  不管一次几步，只要走了a + mb步一定停在入口处，只是一次两步可能永远找不到m！(b = 4 a = 3 a+mb必然是奇数)
        */
        ListNode* fast = head;
        ListNode* slow = head;
        //while(fast != slow)         //不要这样写，，因为初始的时候俩都是head,根本不会进入循环。。。。
        while(true)
        {
            if(fast == nullptr || fast->next == nullptr)    //这里顺序也有讲究的。。如果输入一个空链表，访问fast->next是错误的
            return nullptr;

            fast = fast->next->next;
            slow = slow->next;

            if(fast == slow) 
            break;
        }

        fast = head;
        while(slow != fast)
        {
            slow = slow->next;
            fast = fast->next;
        }
        return fast;
    }
};