Day03：链表part01：链表理论基础、203.移除链表元素、 707.设计链表、206.反转链表

之前的博客：https://blog.csdn.net/weixin_43303286/article/details/131720323

链表理论基础

这里记一下java定义链表的方式：

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; }
}

跟cpp差不多，就是cpp是struct，而且构造函数有点不一样

203. 移除链表元素

时时刻刻记得把箭头改成点！！！并且移除链表元素的时候应该记录待删除节点的前一个节点：

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

707.设计链表

跟cpp的逻辑一样的，在里面声明一个内部类，以及虚拟头节点进行操作。

class MyLinkedList {
        class ListNode {
            int val;
            ListNode next;

            ListNode() {
            }

            ListNode(int val) {
                this.val = val;
            }

            ListNode(int val, ListNode next) {
                this.val = val;
                this.next = next;
            }

        }
        private int _size;
        private ListNode _dummy;
        public MyLinkedList() {
            _size = 0;
            _dummy = new ListNode(-1, null);
        }

        public int get(int index) {
            if(index < 0 || index >= _size){
                return -1;
            }
            ListNode p = _dummy.next;
            while(index > 0){
                index--;
                p = p.next;
            }
            return p.val;
        }

        public void addAtHead(int val) {
            ListNode p = new ListNode(val, null);
            p.next = _dummy.next;
            _dummy.next = p;
            _size++;
        }

        public void addAtTail(int val) {
            ListNode p = new ListNode(val, null);
            ListNode cur = _dummy;
            while(cur.next != null){
                cur = cur.next;
            }
            cur.next = p;
            _size++;
        }

        public void addAtIndex(int index, int val) {
            if(index < 0 || index > _size){
                return;
            }
            ListNode p = new ListNode(val, null);
            ListNode cur = _dummy;
            while(index > 0){
                index--;
                cur = cur.next;
            }
            p.next = cur.next;
            cur.next = p;
            _size++;

        }

        public void deleteAtIndex(int index) {
            if(index < 0 || index >= _size){
                return;
            }
            ListNode cur = _dummy;
            while(index > 0){
                index--;
                cur = cur.next;
            }
            cur.next = cur.next.next;
            _size--;
        }
    }

206. 反转链表

没啥好说的，递归法还是不会：

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

原文地址：https://blog.csdn.net/weixin_43303286/article/details/136554413

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