代码随想录算法训练营第三天 203.移除链表元素 707.设计链表 206.翻转链表

学习任务;掌握链表和数组的区别以及用法

题目链接203. 移除链表元素

设置虚拟头节点

/**
 * 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) {
        if(head == null){
            return head;
        }
        ListNode dummy = new ListNode(-1,head);
        ListNode pre = dummy;
        ListNode cur = head;
        while(cur!=null){
            if(cur.val==val){
                pre.next=cur.next;
            }else{
                pre=cur;
            }
            cur=cur.next;
        }
        return dummy.next;
    }
}

题目链接707. 设计链表

class MyLinkedList {
    int size;
    ListNode head;
    public MyLinkedList() {
        size=0;
        head = new ListNode(0);
        
    }
    
    public int get(int index) {
        if(index < 0 || index >= size){
            return -1;
        }
        ListNode cur = head;
        for(int i=0; i<=index; i++){
            cur = cur.next;
        }
        return cur.val;
    }
    
    public void addAtHead(int val) {
        addAtIndex(0,val);
    }
    
    public void addAtTail(int val) {
        addAtIndex(size,val);
    }
    
    public void addAtIndex(int index, int val) {
        if (index > size) {
            return;
        }
        if (index < 0) {
            index = 0;
        }
        size++;
        //找到要插入节点的前驱
        ListNode pred = head;
        for (int i = 0; i < index; i++) {
            pred = pred.next;
        }
        ListNode toAdd = new ListNode(val);
        toAdd.next = pred.next;
        pred.next = toAdd;
    }

    //删除第index个节点
    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size) {
            return;
        }
        size--;
        if (index == 0) {
            head = head.next;
	    return;
        }
        ListNode pred = head;
        for (int i = 0; i < index ; i++) {
            pred = pred.next;
        }
        pred.next = pred.next.next;
    }
    
}

题目链接206. 反转链表


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

    }
}

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