移除链表元素

[题目](203. 移除链表元素)

思路

移除链表元素实际就是更换节点的尾节点，跳过需要移除的元素节点就行。
可以设置一个虚拟头节点，这样原链表的所有节点就可以按照同意的方式来进行移除

实现代码

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode* dummyHead = new ListNode(0); // 虚拟头结点
        dummyHead->next = head; // 将虚拟头节点指向head，这样方便后面删除
        ListNode* cur = dummyHead;
        while (cur->next != NULL) {
            if (cur->next->val == val) {
                ListNode* tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            } else {
                cur = cur->next;
            }
        }
        head = dummyHead->next;
        delete dummyHead;
        return head;
    }
};

设计链表

[题目](707. 设计链表)

实现代码

class MyLinkedList {
public:
    struct LinkedNode {
        int val;
        LinkedNode* next;
        LinkedNode(int val): val(val), next(nullptr) {}
    }

    MyLinkedList() {
        _dummyHead = new LinkedNode(0);
        _size = 0;
    }
    
    int get(int index) {
        if (index > (size - 1) || index < 0) {
            return -1;
        }
        LinkedNode* cur = _dummyHead->next;
        while (index--) {
            cur = cur->next;
        }
        return cur->val;
    }
    
    void addAtHead(int val) {
        LinkedNode* newNode = new LinkedNode(val);
        newNode->next = _dummyHead->next;
        _dummyHead->next = newNode;
        _size++;
    }
    
    void addAtTail(int val) {
        LinkedNode* newNode = new LinkedNode(val);
        LinkedNode* cur = _dummyHead;
        while (cur->next != nullptr) {
            cur = cur->next;
        }
        cur->next = newNode;
        _size++;
    }
    
    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++;
    }
    
    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;
        _size--;
    }
private:
    LinkedNode* _dummyHead;
    int _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);
 */
};

翻转链表

[题目](206. 反转链表)

思路

可以使用双指针来处理，定义前后指针进行翻转

代码实现

/**
 * 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* temp;
        ListNode* cur = head;
        ListNode* pre = NULL;
        while (cur) {
            temp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
};