Question

I need help completing this with C++

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
  
}
};

Answer 1

Iterative Solution

class Solution{

public:

ListNode* reverseList( ListNode* head) {

ListNode* cur = head;

ListNode* after = NULL;

ListNode* prev = NULL;

while( cur)

{

after = cur->next;

cur -> next = prev;

prev = cur;

cur = after;

}

return prev;

}

};

*******************************************************************************************************************************

Recursive Solution

class Solution{

public:

ListNode* reverseList( ListNode* head) {

if( head == NULL)

return NULL;

  if( head->next == NULL)

return head;

ListNode* first = head;

ListNode* rest;

rest = reverseList( head -> next);

first -> next -> next = first;

first ->next = NULL;

head = rest;

return head;

}

};