206. Reverse Linked List

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

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

Runtime: 8 ms, faster than 43.08% of C++ online submissions for Reverse Linked List.

/**
 * 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* result = NULL;
        while (head) {
            ListNode* next = head -> next;
            head -> next = result;
            result = head;
            head = next;
        }
        return result;
    }
};

Runtime: 76 ms, faster than 89.64% of JavaScript online submissions for Reverse Linked List.
Memory Usage: 40.5 MB, less than 66.21% of JavaScript online submissions for Reverse Linked List.

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    let slow = null;
    let p = head;
    let temp;

    while(p) {
        temp = p.next;
        p.next = slow;
        slow = p;
        p = temp;
    }
    return slow;
};