leetcode-206

206. Reverse Linked List

Description

Reverse a singly linked list.

Example:

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Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

Analyse

反转单链表

尝试用迭代和递归两种方法来做

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        1  ->   2   -> 3 -> 4 -> 5
pre -> cur -> next

Code

迭代

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ListNode* reverseList(ListNode* head)
{
    ListNode* pre = NULL;
    ListNode* cur = head;
    ListNode* next;

    while (cur)
    {
        next = cur->next;
        cur->next = pre;
        pre = cur;
        cur = next;
    }

    return pre;
}

递归

找到递归子问题

对每个单链表，每次将最后两个 node 的指向反转

举个例子

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1->2->3->4->5
1->2->3->4<-5
1->2->3<-4<-5
1->2<-3<-4<-5
1<-2<-3<-4<-5
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public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        // lastNode是未逆序的最后一个节点
        ListNode lastNode = reverseList(head.next);
        // 此时的head是lastNode前面一个节点
        head.next.next = head;
        head.next = null;

        return lastNode;
    }