Reverse a singly linked list.
將第一個節點作為當前節點，將當前節點的下一個存下來，當前節點的下一個節點指向前一個節點，當前節點作為前一個節點，將剛才存下的下一個節點作為當前節點。

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    var prev = null;
    while (head!==null) {
        var next = head.next;
        head.next = prev;
        prev = head;
        head = next;
    }
    return prev;
};