206. Reverse Linked List (easy). !!

Q:

Given the head of a singly linked list, reverse the list, and return the reversed list.

 

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

 

Constraints:

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

A:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
用dummy header
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode header;
        while (head) {
            auto tmp = head;
            head = head->next;
            tmp->next = header.next;
            header.next = tmp;
        }
        return header.next;
    }
};


或者把 新的head，不用 dummy header， 而是指向最新的head. 只是尽量晚赋值
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* newHead = nullptr;
        while (head) {
            ListNode* nextNode = head->next;
            head->next = newHead;
            newHead = head;
            head = nextNode;
        }
        return newHead;
    }
};



**********Mistakes*************
ListNode *header(0);  ---------> this grammar is WRONG. 
It initializes header as a null pointer (nullptr) because:
header is a pointer (ListNode*).
0 is an integer literal, which implicitly converts to nullptr in C++ when assigned to a pointer.
Thus header is equivalent to 
          ListNode* header = nullptr; // Explicitly setting it to nullptr

This means header->next is an invalid operation because header is nullptr.


Correct Way to Create a Dummy Node
If you meant to create an actual ListNode object, you should write:
      ListNode dummy(0);  // Correct: Creates an actual ListNode

or use dynamic allocation:

      ListNode* header = new ListNode(0); // Allocates a real ListNode on the heap