LeetCode reverse-linked-list

反转一个单链表。

示例: > 输入: 1->2->3->4->5->NULL > 输出: 5->4->3->2->1->NULL

进阶: 你可以迭代或递归地反转链表。你能否用两种方法解决这道题?

解答:

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/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList1(ListNode* head) {
ListNode *pre = nullptr;
ListNode *cur = head;

while (cur) {
ListNode *p = cur->next;
cur->next = pre;
pre = cur;
cur = p;
}
return pre;
}

ListNode* reverseList2(ListNode* head) {
return reverse(nullptr, head);
}

ListNode* reverse(ListNode* pre, ListNode* cur) {
if (!cur) return pre;
ListNode *next = cur->next;
cur->next = pre;
return reverse(cur, next);
}
};

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