206. Reverse Linked List

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 = []

CODE use C++:

/**
 * 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) {}
 * };
*/
class Solution {
    
    public:
    
    
    ListNode* reverseList(ListNode* head) {
        int len = 0, now = 0, end = 0;
        ListNode *tmp = head;
        if(head == NULL){
            return head;
        }
        while(tmp->next != NULL){
            tmp = tmp->next;
            end++;
            len++;
        }
        int tmp_int = head->val;
        head->val = tmp->val;
        tmp->val = tmp_int;
        now++;
        end--;
        ListNode *tmp_head = head;
        while(end>now){
            int tmp_now = end;
            tmp = head;
            while(tmp_now!=0){
                tmp = tmp->next;
                tmp_now--;
            }
            tmp_head = tmp_head->next;
            int tmp_int = tmp_head->val;
            tmp_head->val = tmp->val;
            tmp->val = tmp_int;
            end--;
            now++;
        }
        return head;
        
        
    }
    
   
};

Runtime: 28 ms, faster than 57.91% of C++ online submissions for Reverse Linked List.

Memory Usage: 8.3 MB, less than 75.89% of C++ online submissions for Reverse Linked List.