随想录day3:203.移除链表元素|707.设计链表 |206.反转链表

203.移除链表元素

方法一：直接遍历，永远记得处理head, 删除链表必须有前驱。

/**
 * 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) {
        while(head != null && head.val == val){
            head = head.next;
        }

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

方法二：用dummy 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; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        ListNode dummy = new ListNode();
        dummy.next = head;

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

707.设计链表

class MyLinkedList {
    Node head;
    public MyLinkedList() {
        head = new Node(-1);
    }
    
    public int get(int index) {
        Node curr = head.next;
        for(int i=0;i<index && curr != null;i++) 
            curr = curr.next;
        return curr==null?-1:curr.val;
    }
    
    public void addAtHead(int val) {
        Node node = new Node(val);
        node.next = head.next;
        head.next = node;
    }
    
    public void addAtTail(int val) {
        Node curr = head;
        while(curr.next != null)
            curr = curr.next;
        Node node = new Node(val);
        curr.next = node;
    }
    
    public void addAtIndex(int index, int val) {
        Node prev = head;
        for(int i=0;i<index && prev != null;i++)
            prev = prev.next;
        if(prev == null) return;
        Node node = new Node(val);
        node.next = prev.next;
        prev.next = node;
    }
    
    public void deleteAtIndex(int index) {
        Node prev = head;
        for(int i=0;i<index && prev != null;i++)
            prev = prev.next;
        if(prev != null && prev.next != null)
            prev.next = prev.next.next;
    }

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

206.反转链表 =>保存断开的节点

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

    }
}