C++链表反转

- 反转链表

1，2，3，4，5
5，4，3，2，1

方法一（迭代法）：

#include <iostream>

using namespace std;

//定义链表节点
struct ListNode {
        int val;
        ListNode* next;
      ListNode(int x) : val(x), next(nullptr) {}

};
    //迭代法解题
ListNode* reverseList (ListNode* head) {
        ListNode* prev = nullptr;
        ListNode* curr = head;
        while(curr != NULL){
            ListNode* nextTemp = curr -> next;
            curr -> next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }
 void printList(ListNode* head){
    ListNode* curr = head;
    while(curr != nullptr){
        cout << curr->val << ',';
        curr = curr->next;
    }
    cout << endl;
 }   
ListNode* creatList(){
    int val;
    ListNode* head = nullptr;
    ListNode* tail = nullptr;
    while(true){
        cin >> val;
        if(val == -1) break;
        ListNode* newNode = new ListNode(val);
        if(head == nullptr){//输入头结点
            head = newNode;
            tail = head;
        }
        else{
            tail->next = newNode;//插入尾结点
            tail = newNode;
        }
    }
    return head;
}

int main() {
    //创建链表
    // 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);

    //手动输入链表
    ListNode* head = creatList();

    cout << "Original list: ";
    printList(head);  // 输出原链表
    
    ListNode* ReserveListHead = reverseList(head);
    printList(ReserveListHead);

    return 0;
}

方法二（递归法）：

#include <iostream>

using namespace std;

//定义链表节点
struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(nullptr) {}

};
    //递归法解题
ListNode* reverseList (ListNode* head) {
    if(head == nullptr || head->next == nullptr){
        return head;
    } 
    ListNode* newHead = reverseList(head->next);
    head->next->next = head;
    head->next = nullptr;
    return newHead;

}
 void printList(ListNode* head){
    ListNode* curr = head;
    while(curr != nullptr){
        cout << curr->val << ' ';
        curr = curr->next;
    }
    cout << endl;
 }   
// ListNode* creatList(){
//     int val;
//     ListNode* head = nullptr;
//     ListNode* tail = nullptr;
//     while(true){
//         cin >> val;
//         if(val == -1) break;
//         ListNode* newNode = new ListNode(val);
//         if(head == nullptr){//输入头结点
//             head = newNode;
//             tail = head;
//         }
//         else{
//             tail->next = newNode;//插入尾结点
//             tail = newNode;
//         }
//     }
//     return head;
// }

int main() {
    //创建链表
    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);

    //手动输入链表
    // ListNode* head = creatList();

    cout << "Original list: ";
    printList(head);  // 输出原链表
    
    ListNode* ReserveListHead = reverseList(head);
    printList(ReserveListHead);

    return 0;
}