Task

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?

Solution

  • 定义三个指针prev、curr、next，依次修改
  • 每次迭代完成后，链表分为两部分，左侧的头部是prev，右侧的头部是curr和next
  • 最终结束时curr和next指向原来的空指针，prev指向翻转后的链表
  • 时间复杂度：只遍历一次，O(n)
  • 空间复杂度：只用了3个指针，O(1)
  • 实现：
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// Runtime: 4 ms, faster than 97.48% of C++ online submissions for Reverse Linked List.
// Memory Usage: 8 MB, less than 100.00% of C++ online submissions for Reverse Linked List.
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        //处理空链表
        if(!head) return head;
        //定义三个指针
        ListNode *prev=nullptr,*curr=head,*next=head->next;
        while(curr){
            next=curr->next;
            curr->next=prev;
            prev=curr;
            curr=next;
        }
        return prev;
    }
};