206. Reverse Linked List

iterative O(n)

/**
 * 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, *curr = head;
        while (curr) {
            auto succ = curr->next;
            curr->next = prev;
            prev = curr;
            curr = succ;
        }
        return prev;
    }
};

recursive O(n) time

/**
 * 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;
        auto res = reverseList(head->next);
        head->next->next = head;
        head->next = nullptr;
        return res;
    }
};