206. Reverse Linked List

Description

Reverse a singly linked list.

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

Solution

Solution 1

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * Iterative version
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    "use strict";
    if (!head) {
        return head;
    }
    let prev = null;
    while(head) {
        let tmp = head.next;
        head.next = prev;
        prev = head;
        head = tmp;
    }
    return prev;
};

Solution 2

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * Recursive version
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    "use strict";
    function reverse(head, prev) {
        if (!head) {
            return prev;
        }
        let next = head.next;
        head.next = prev;
        return reverse(next, head);
    }
    return reverse(head, null);
};