206. Reverse Linked List

Problem

Tags: Linked List, Recursion

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

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

Constraints:

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

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

Code

JS

// 206. Reverse Linked List (7/20/53737)
// Runtime: 76 ms (62.01%) Memory: 41.11 MB (92.76%) 

/**
 * 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}
 */
function reverseList(head) {
    return recv(head, null);

    function recv(node, prev) {
        if (node) {
            const next = node.next;
            node.next = prev;
            return recv(next, node);
        }
        return prev;
    }
}