链式存储结构

206. Reverse Linked List

How to exchange the nodes without changing their values?
from the side to another: u could see[pre, cur, cur->next]
it’s easy to know when “cur” gets NULL, the “pre” is at the end of linked list.
we could store “cur->next” in temp then makes “cur” at front of “pre”
so now “pre” turned to “cur” and “cur” turned to the temp’s value(used to be “cur->next”)

ListNode* reverseList(ListNode* head) {
        ListNode* pre=NULL;
        ListNode* cur=head;
        while(cur!=NULL){
            auto t=cur->next;
            cur->next=pre;
            pre=cur;
            cur=t; 
        }
        return pre;
}