代码随想录训练营第三天｜203.移除链表元素, 707.设计链表, 206.反转链表

第三天是链表，要注意的是可以创建虚拟头节点来进行链表操作。

class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode temp = new ListNode(-1);
        temp.next = head;
        ListNode cur = temp;

        while(cur.next!=null){
            if(cur.next.val == val){
                cur.next = cur.next.next;
            }
            else{
                cur = cur.next;
            }
        }
        return temp.next;
    }
}

链表中基础的删除节点，创建一个虚拟节点，虚拟节点的next 为head，然后不断next，如果这个虚拟节点的next为需要删除的数值，那么就不再指向这个节点。

707. 设计链表

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

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

}
class MyLinkedList {
    private int size;
    private ListNode head;
    public MyLinkedList() {
        size = 0;
        head = new ListNode(0);
    }
    
    public int get(int index) {
        if(index < 0 || index >= size){
            return -1;
        }
        ListNode temp = head;
        for(int i = 0;i<= index; i++){
            temp = temp.next;
        }
        return temp.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 < 0 || index > size){
            return; 
        }
        size ++;
        ListNode temp = head;
        for(int i = 0; i<index; i++){
            temp = temp.next;
        }
        ListNode cur = new ListNode(val);
        cur.next = temp.next;
        temp.next = cur;
    }
    
    public void deleteAtIndex(int index) {
        if(index < 0 || index>= size){
            return;
        }
        size--;
        ListNode temp = head;
        for(int i = 0;i < index; i++){
            temp = temp.next;
        }
        temp.next = temp.next.next;
    }
}

需要先创建一个ListNode class，然后再根据题意，实现链表的查和增删功能

206. 反转链表

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode cur = head;
        ListNode pre = null;
        ListNode post = null;

        while(cur != null){
            post = cur.next;
            cur.next = pre;
            pre = cur;
            cur = post;
        }
        return pre;
    }
}