代码随想录二刷day03

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前言

一、力扣203. 移除链表元素

/**
 * 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; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode list = new ListNode();
        list.next = head;
        ListNode p = head, pre = list;
        while(p != null){
            if(p.val == val){
                pre.next = p.next;
                p = p.next;
            }else{
                pre = p;
                p = p.next;
            }
        }
        return list.next;
    }
}

二、力扣707. 设计链表

class MyLinkedList {
    int size;
    DLinkList head, tail;

    public MyLinkedList() {
        this.size = 0;
        this.head = new DLinkList(0);
        this.tail = new DLinkList(0);
        this.head.next = this.tail;
        this.tail.prev = this.head;
    }
    
    public int get(int index) {
        if(index < 0 || index >= this.size){
            return -1;
        }
        DLinkList p = head;
        int i = 0;
        while(i <= index){
            i ++;
            p = p.next;
        }
        return p.val;
    }
    
    public void addAtHead(int val) {
        addAtIndex(0, val);
    }
    
    public void addAtTail(int val) {
        addAtIndex(this.size, val);
    }
    
    public void addAtIndex(int index, int val) {
        if(index < 0 || index > this.size){
            return ;
        }
        DLinkList node = new DLinkList(val);
        int i = 0;
        DLinkList p = head;
        while(i < index){
            i ++;
            p = p.next;
        }
        node.next = p.next;
        p.next.prev = node;
        node.prev = p;
        p.next = node;
        this.size ++;
    }
    
    public void deleteAtIndex(int index) {
        if(index < 0 || index > this.size-1){
            return;
        }
        DLinkList p = head;
        int i = 0;
        while(i < index){
            i ++;
            p = p.next;
        }
        p.next.next.prev = p;
        p.next = p.next.next;
        this.size --;
    }
}
class DLinkList{
    int val;
    DLinkList prev;
    DLinkList next;
    DLinkList(int val){
        this.val = val;
        this.prev = null;
        this.next = null;
    }
}

/**
 * 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);
 */

三、力扣206. 反转链表

头插法

/**
 * 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; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode node = new ListNode(-1, null);
        ListNode p = head, s;
        while(p != null){
            s = p; p = p.next;
            s.next = node.next;
            node.next = s;
        }
        return node.next;
    }
}

递归

/**
 * 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; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        return reverse(null, head);
    }
    public ListNode reverse(ListNode prev, ListNode cur){
        if(cur == null){
            return prev;
        }
        ListNode p = cur.next;
        cur.next = prev;
        return reverse(cur, p);
    }
}

双指针法

/**
 * 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; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null){
            return head;
        }
        ListNode prev = null;
        ListNode cur = head;
        ListNode temp ;
        while(cur != null){
            temp = cur.next;
            cur.next = prev;
            prev = cur;
            cur = temp;
        }
        return prev;
    }
}