206. Reverse Linked List

Given the head of a singly linked list, reverse the list, and return the reversed list.

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

• iterative solution
• Time : O(n)
• Space : O(1)

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

• recursive solution
• Time : O(n)
• Space : O(1)

/**
 * 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 {
    ListNode* solution(ListNode* parent, ListNode* child) {
        if(!child) return parent;
        ListNode* res = solution(child, child->next);
        child->next = parent;
        parent->next = NULL;
        return res;
    }
public:
    ListNode* reverseList(ListNode* head) {
        return !head ? head : solution(head, head->next);
    }
};