单链表反转

/**
 * 边界条件，当链表为null或链表中仅有一个节点时，不需要反转
 */
var reverseList = function(head) {
    if (!head || !head.next) return head
    let prev = null, cur = head;
    while(cur) {
        // 记录下一个节点
        var next = cur.next;
        cur.next = prev;
        prev = cur
        cur = next
    }
    head = prev;
    return head;
};