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reverseLinkedList.cpp
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reverseLinkedList.cpp
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#include <bits/stdc++.h>
using namespace std;
class node
{
public:
int data;
node *next;
node(int val)
{
data = val;
next = NULL;
}
};
// iterative approach
node *reverse(node *&head)
{
node *prevPtr = NULL;
node *currPtr = head;
node *nextPtr;
while (currPtr != NULL)
{
nextPtr = currPtr->next;
currPtr->next = prevPtr;
prevPtr = currPtr;
currPtr = nextPtr;
}
return prevPtr; // new head
}
// recursive approach
node *reverseRecursive(node *&head)
{
// base condition
if (head == NULL || head->next == NULL)
{
return head;
}
node *newHead = reverseRecursive(head->next);
head->next->next = head;
head->next = NULL;
return newHead;
}
node *reverseK(node *&head, int k)
{
node *prevPtr = NULL;
node *currPtr = head;
node *nextPtr;
int count = 0;
while (currPtr != NULL && count < k)
{
nextPtr = currPtr->next;
currPtr->next = prevPtr;
prevPtr = currPtr;
currPtr = nextPtr;
count++;
} // k nodes
if (nextPtr != NULL)
{
head->next = reverseK(nextPtr, k);
}
return prevPtr;
}
void insertAtTail(node *&head, int val)
{
node *newNode = new node(val); // created a new node;
if (head == NULL)
{
head = newNode;
return;
// here you can simply call insertAtBeginning
}
node *temp = head;
while (temp->next != NULL)
{
temp = temp->next;
}
temp->next = newNode;
// as our new node already points to NULL in its next so we simply do nothing
}
void display(node *head)
{
if (head == NULL)
{
cout << "NULL" << endl;
return;
}
node *temp = head;
while (temp != NULL)
{
cout << temp->data << "->";
temp = temp->next;
}
cout << "NULL" << endl;
}
int main()
{
node *head = NULL;
insertAtTail(head, 1);
insertAtTail(head, 23);
insertAtTail(head, 12);
insertAtTail(head, 10);
display(head);
// node *newhead = reverseRecursive(head);
int k = 2;
node *newHead = reverseK(head, k);
display(newHead);
}