反转链表

头插法

/**
 * 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) {
        ListNode* dummy = new ListNode(-1);
        dummy->next = NULL;
        ListNode* cur = dummy;
        while (head) {
            ListNode* node = new ListNode(head->val);
            node->next = cur->next;
            cur->next = node;
            head = head->next;
        }
        return dummy->next;
    }
};

迭代法

/**
 * 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) {
        ListNode* prev = nullptr;
        ListNode* cur = head;
        while (cur) {
            ListNode* next = cur->next;
            cur->next = prev;
            prev = cur, cur = next;
        }
        return prev;
    }
};

递归法

/**
 * 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) {
        if (!head || !head->next) return head;
        ListNode* tail = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return tail;
    }
};