代码随想录-链表

203. 移除链表元素 easy

简单题，看看CPP的指针的写法就好

/**
 * 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) {}
 * };
 */

// 迭代法
class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode * h=new ListNode();
        h->next = head;
        ListNode *t = h;
        while(t->next != nullptr){
            if(t->next->val == val){
                ListNode * temp = t->next;
                t->next = t->next->next;
                delete temp;
            }else
                t = t->next;
        }
        return h->next;
    }
};

// 递归法
class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (head == nullptr) {
            return head;
        }
        head->next = removeElements(head->next, val);
        return head->val == val ? head->next : head;
    }
};

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */

// 迭代法
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head == null){
            return head;
        }
        ListNode hh = new ListNode();
        ListNode h = hh;
        
        h.next = head;
        while(h.next != null){
            if(h.next.val == val){
                h.next = h.next.next;
            }else{
                h = h.next;
            }
        }
        return hh.next;
    }
}
// 递归法
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if(head == null){
            return head;
        }
        head.next = removeElements(head.next, val);
        return head.val == val? head.next:head;
    }
}

707.设计链表 middle

考察基础，并不难，主要理解在cpp在class中创建struct或者java在class中创建class。

class MyLinkedList {
public:
    struct LinkedNode {
        int val;
        LinkedNode  * next;
        LinkedNode () : val(0), next(nullptr) {}
        LinkedNode (int x) : val(x), next(nullptr) {}
        LinkedNode (int x, LinkedNode  *next) : val(x), next(next) {}
    };

    MyLinkedList() {
        preHead = new LinkedNode ();
        _size = 0;
    }
    
    int get(int index) {
        if (index > (_size - 1) || index < 0) {
            return -1;
        }
        LinkedNode  * t = preHead->next;
        while(index--){
            t = t->next;
        }
        return t->val;
    }
    
    void addAtHead(int val) {
        LinkedNode  * ins = new LinkedNode(val,preHead->next);
        preHead->next = ins;
        _size++;
    }
    
    void addAtTail(int val) {
        LinkedNode  * ins = new LinkedNode(val,nullptr);
        LinkedNode* cur = preHead;
        while(cur->next != nullptr){
            cur = cur->next;
        }
        cur->next = ins;
        _size++;
    }
    
    void addAtIndex(int index, int val) {
        if(index > _size) return;
        if(index < 0) index = 0;
        LinkedNode  * ins = new LinkedNode(val);
        LinkedNode  * t = preHead;
        while(index--){
            t = t->next;
        }
        ins->next = t->next;
        t->next = ins;
        _size++;
        return;
    }
    
    void deleteAtIndex(int index) {
        if(index < 0 || index >= _size) return;
        LinkedNode  * t = preHead;
        while(index--){
            t = t->next;
        }
        LinkedNode* tmp = t->next;
        t->next = t->next->next;
        _size--;
        delete tmp;
        return;
    }

private:
    int _size;
    LinkedNode* preHead;

};

//单链表
class MyLinkedList {

    class ListNode {
        int val;
        ListNode next;
        ListNode(int val) {
            this.val=val;
        }
    }
    //size存储链表元素的个数
    private int size;
    //注意这里记录的是虚拟头结点
    private ListNode head;

    //初始化链表
    public MyLinkedList() {
        this.size = 0;
        this.head = new ListNode(0);
    }

    //获取第index个节点的数值，注意index是从0开始的，第0个节点就是虚拟头结点
    public int get(int index) {
        //如果index非法，返回-1
        if (index < 0 || index >= size) {
            return -1;
        }
        ListNode cur = head;
        //第0个节点是虚拟头节点，所以查找第 index+1 个节点
        for (int i = 0; i <= index; i++) {
            cur = cur.next;
        }
        return cur.val;
    }

    public void addAtHead(int val) {
        ListNode newNode = new ListNode(val);
        newNode.next = head.next;
        head.next = newNode;
        size++;

        // 在链表最前面插入一个节点，等价于在第0个元素前添加
        // addAtIndex(0, val);
    }

    
    public void addAtTail(int val) {
        ListNode newNode = new ListNode(val);
        ListNode cur = head;
        while (cur.next != null) {
            cur = cur.next;
        }
        cur.next = newNode;
        size++;

        // 在链表的最后插入一个节点，等价于在(末尾+1)个元素前添加
        // addAtIndex(size, val);
    }

    // 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
    public void addAtIndex(int index, int val) {
        if (index < 0 || index > size) {
            return;
        }

        //找到要插入节点的前驱
        ListNode pre = head;
        for (int i = 0; i < index; i++) {
            pre = pre.next;
        }
        ListNode newNode = new ListNode(val);
        newNode.next = pre.next;
        pre.next = newNode;
        size++;
    }

    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size) {
            return;
        }
        
        //因为有虚拟头节点，所以不用对index=0的情况进行特殊处理
        ListNode pre = head;
        for (int i = 0; i < index ; i++) {
            pre = pre.next;
        }
        pre.next = pre.next.next;
        size--;
    }
}