b->next = a是改变b的next指针的指向，使其指向a
等于号左边的是被改变的（因为被赋予了新值）
很容易混淆！！！

由反转链表这道题产生的疑惑！

/
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode next;
* ListNode(int x) : val(x), next(NULL) {}
* };
/
class Solution {
public:
ListNode reverseList(ListNode head) {
if(!head || !head->next) return head;
auto a = head, b = a->next;
while(b){
auto c = b->next;
b->next = a;
a = b, b = c;
}
head->next = NULL;
return a;
}
};