【easy】206. Reverse Linked List 链表反转

链表反转，一发成功~

/**
 * 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) {
        //就地反转
        if(head == NULL || head ->next == NULL) 
            return head;
        ListNode* pre = head;
        ListNode* cur = head->next;
        ListNode* nxt = NULL;
        
        while (cur){
            nxt = cur->next;
            cur->next = pre;
            pre = cur;
            cur = nxt;
        }
        head->next = NULL;
        head = pre;
        return head;
    }
};