206. Reverse Linked List

Iteration

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

• T: $O(N)$
• S: $O(1)$

Recursion

/**
 * 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)
    {
        if(!head || !head->next) return head;

        ListNode* prev = reverseList(head->next);
        // 1 -> 2 <- 3 <- 4 <- 5
        // ^ head

        head->next->next = head;
        head->next = NULL;
        return prev;
    }
};

• T: $O(N)$
• S: $O(N)$