给定单链表的头节点 head ，请反转链表，并返回反转后的链表的头节点。

输入：head = [1,2,3,4,5]
输出：[5,4,3,2,1]

/**
 * 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) {
    var prev = null
    var curr = head

    while(curr) {
        var next = curr.next

        curr.next = prev
        prev = curr
        curr = next
    }

    return prev
};

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/UHnkqh