非递归：
function reverseList(head) {
var prev = null;
while (head) {
var
next = head.
next;
head.
next = prev;
prev = head;
head =
next;
}
return prev;
}
递归：
function reverseList(head) {
if (!head || !head.
next) {
return head;
}
var newHead = reverseList(head.
next);
head.
next.
next = head;
head.
next = null;
return newHead;
}
