leetcode
206.反转链表
双指针迭代法
// Definition for singly-linked list.
// struct ListNode {
// int val;
// ListNode *next;
// ListNode() : val(0), next(nullptr) {}
// ListNode(int x) : val(x), next(nullptr) {}
// ListNode(int x, ListNode *next) : val(x), next(next) {}
// };
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* current = head;
ListNode* pre = nullptr;
while (current){
// 指针换向
ListNode* tmp = current->next;
current->next = pre;
// 后移
pre = current;
current = tmp;
}
return pre;
}
};
递归法
class Solution {
public:
ListNode* reverse(ListNode* current, ListNode* pre){
if (!current) return pre; // 错误 return nullptr;
ListNode*tmp = current->next;
current->next = pre;
return reverse(tmp, current);
}
ListNode* reverseList(ListNode* head) {
return reverse(head, nullptr);
}
};