力扣第206号问题的三种解法

Posted 2022-04-29 *平芜尽处是春山*

解法一、头插法

class Solution 
    public ListNode reverseList(ListNode head) 
         if(head == null || head.next == null) 
            return head;
        
        ListNode newHead = null;
        while(head != null) 
            ListNode node = new ListNode(head.val);
            node.next = newHead;
            newHead = node;
            head = head.next;
        
        return newHead;
    

运行截图：

解法二、原地修改链表

class Solution 
    public ListNode reverseList(ListNode head) 
        if(head == null || head.next == null) 
            return head;
        
        ListNode prev = null;
        ListNode cur = head;
        while(cur != null) 
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        
        return prev;
    

运行截图：

解法三、递归法

class Solution 
    public ListNode reverseList(ListNode head) 
        if(head == null || head.next == null) 
            return head;
        
        ListNode secNode = head.next;
        ListNode newHead = reverseList(head.next);
        secNode.next = head;
        head.next = null;
        return newHead;
    

运行截图：