41-设计链表

自己的做法

算法思想

初始化链表时添加哑节点，使用两个指针dummy和end分别指向链表哑节点和链表尾节点，并使用size变量表示链表长度。

算法实现

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 MyLinkedList {
private:
    ListNode * dummy = nullptr, * end = nullptr;
    int size;
public:
    /** Initialize your data structure here. */
    MyLinkedList() {
        dummy = new ListNode();
        end = dummy;
        size = 0;
    }

    /** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
    int get(int index) {
        if(index < 0 || index >= size) {
            return -1;
        }
        ListNode * cur = dummy;
        //走 index + 1 步
        for(int i = 0; i <= index; i++) {
            cur = cur->next;
        }
        return cur->val;
    }

    /** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */
    void addAtHead(int val) {
        ListNode * node = new ListNode(val);
        //如果是链表为空，则修改end指针
        if(dummy == end) {
            end = node;
        }
        node->next = dummy->next;
        dummy->next = node;
        size++;
    }

    /** Append a node of value val to the last element of the linked list. */
    void addAtTail(int val) {
        ListNode * node = new ListNode(val);
        end->next = node;
        end = end->next;
        size++;
    }

    /** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
    void addAtIndex(int index, int val) {
        if(index > size) {
            return;
        }
        if(index <= 0) {
            //索引小于等于0，添加到头部
            addAtHead(val);
        } else if(index == size) {
            //index == size，则添加到最后
            addAtTail(val);
        } else {
            ListNode * pre = dummy;
            for(int i = 0; i < index; i++) {
                pre = pre->next;
            }
            ListNode * node = new ListNode(val);
            node->next = pre->next;
            pre->next = node;
            size++;
        }
    }

    /** Delete the index-th node in the linked list, if the index is valid. */
    void deleteAtIndex(int index) {
        if(index < 0 || index >= size) {
            return;
        }
        ListNode * pre = dummy;
        for(int i = 0; i < index; i++) {
            pre = pre->next;
        }
        // 如果要删除节点为尾节点，则要修改end指针
        if(index == size - 1) {
            end = pre;
        }
        pre->next = pre->next->next;
        size--;
    }
};

性能分析

链表长度为N。

时间复杂度：

​ get:$O(k)$，k为索引。

​ addAtHead：$O(1)$。

​ addAtTail:$O(1)$。

​ addAtIndex：$O(k)$。

​ deleteAtIndex：$O(k)$。