4.写程序将一个单链表翻转。 例如：1->2->3->4->5 翻转后为：5->4->3->2->1.

好的，以下是单链表翻转的程序示例：

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

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = NULL;
        ListNode* curr = head;
        while (curr != NULL) {
            ListNode* next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
};

使用方法：

ListNode* head = new ListNode(1);
head->next = new ListNode(2);
head->next->next = new ListNode(3);
head->next->next->next = new ListNode(4);
head->next->next->next->next = new ListNode(5);

Solution solution;
ListNode* reversed = solution.reverseList(head);

while (reversed != NULL) {
    cout << reversed->val << " ";
    reversed = reversed->next;
}

输出结果为：

5 4 3 2 1