NO15、反转链表（贼经典，建议多多刷，不开玩笑，要闭着眼睛写出来那种）

15、反转链表

输入一个链表，反转链表后，输出新链表的表头。

示例1
输入

{1,2,3}

返回值

{3,2,1}

很好的解答

https://blog.csdn.net/qq_42351880/article/details/88637387

1、头插法 很经典的做法啊

struct ListNode {
    int val;
    struct ListNode* next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
}; 

ListNode* ReverseList(ListNode* pHead) {

    struct ListNode* Head = NULL;
    struct ListNode* node = (ListNode*)malloc(sizeof(struct ListNode));

    while (pHead != nullptr) {
        node = pHead;
        pHead = pHead->next;

        node->next = Head;
        Head = node;
    }
    return Head;
}

void test02()
{
    ListNode* head = (ListNode*)malloc(sizeof(ListNode));
    head->val = 1;

    ListNode* node1 = (ListNode*)malloc(sizeof(ListNode));
    node1->val = 2;

    ListNode* node2 = (ListNode*)malloc(sizeof(ListNode));
    node2->val = 3;

    ListNode* node3 = (ListNode*)malloc(sizeof(ListNode));
    node3->val = 4;

    head->next = node1;
    node1->next = node2;
    node2->next = node3;
    node3->next = nullptr;

    auto node = ReverseList(head);
    while(node!=nullptr){

        cout << node->val << endl;
        node = node->next;
    }
    }

2、第二种方法，借助三个结点进行不断更替