关于反向链表的问题（leetcode 206）

Question

我知道我的代码完全错误，但我不知道我哪里做错了，

谁能指出并解释我做错了什么？

public ListNode reverseList(ListNode head) {
    if (head == null) {
        return head;
    }
    
    ListNode prev = null;
    ListNode current = head;
    ListNode nextNode = head.next;
    
    while (nextNode != null) {
        prev = current;
        current = nextNode;
        current.next = prev;
        nextNode = nextNode.next;
        System.out.println(nextNode.val);
    }
    
    return current;
}

Answer 1

变化：

1. head.next = null; // 使列表的结尾为空

2. current.next = prev; // current 是对节点的引用，所以对它的更改也会改变引用 nextNode 的节点

   public ListNode reverseList(ListNode head) {
        if (head == null) {
            return head;
        }
   
        ListNode prev = null;
        ListNode current = head;
        ListNode nextNode = head.next;
        head.next = null;   // to make the end of the list as null
   
        while (nextNode != null) {
            prev = current;
            current = nextNode;
            nextNode = nextNode.next;   // first get next node, before ...
            current.next = prev;        // ... overwriting it here
            // System.out.println(nextNode.val);
        }
   
        return current;
    }
   

Answer 2

替代方案

我们可以只说while head != null，然后使用prev 节点将其反转，最后我们将返回prev。这会更容易：

public final class Solution {
    public static final ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode nextNode;

        while (head != null) {
            nextNode = head.next;
            head.next = prev;
            prev = head;
            head = nextNode;
        }

        return prev;
    }
}