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.反转链表

203.移除链表

/**
 * 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 dummyHead = new ListNode();
        dummyHead.next=head;
        ListNode curr = dummyHead;
        while(curr.next != null){
            if(curr.next.val == val){
                curr.next = curr.next.next;
            }else{
                curr=curr.next;
            }
        }
        return dummyHead.next;
    }
}
 

707.设计链表

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

    public ListNode() {
    }
}

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 curr = head;
        for (int i = 0; i <= index ; i++) {
            curr = curr.next;
        }
        return curr.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++;
        ListNode curr = head;
        for (int i = 0; i < index; i++) {
            curr = curr.next;
        }
        ListNode newNode = new ListNode(val);
        newNode.next = curr.next;
        curr.next = newNode;
    }
    
    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size) {
            return;
        }
        size--;
        if(index == 0){
            head = head.next;
            return;
        }

        ListNode curr = head;
        for (int i = 0; i < index ; i++) {
            curr =curr.next;
        }

        curr.next=curr.next.next;
    }
}

/**
 * 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);
 */

206.反转链表

/**
 * 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 null;
        }
        if(head.next == null){
            return head;
        }
        ListNode pre = null, cur = head,tmp;
        while(cur != null){
            tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }
        return pre;
    }
}