## Write a function to reverse a linked list

**Iterative Method**

Iterate trough the linked list. In loop, change next to prev, prev to current and current to next.

**Implementation of Iterative Method**

#include<stdio.h> #include<stdlib.h> /* Link list node */ struct node { int data; struct node* next; }; /* Function to reverse the linked list */ static void reverse(struct node** head_ref) { struct node* prev = NULL; struct node* current = *head_ref; struct node* next; while (current != NULL) { next = current->next; current->next = prev; prev = current; current = next; } *head_ref = prev; } /* Function to push a node */ void push(struct node** head_ref, int new_data) { /* allocate node */ struct node* new_node = (struct node*) malloc(sizeof(struct node)); /* put in the data */ new_node->data = new_data; /* link the old list off the new node */ new_node->next = (*head_ref); /* move the head to point to the new node */ (*head_ref) = new_node; } /* Function to print linked list */ void printList(struct node *head) { struct node *temp = head; while(temp != NULL) { printf("%d ", temp->data); temp = temp->next; } } /* Drier program to test above function*/ int main() { /* Start with the empty list */ struct node* head = NULL; push(&head, 20); push(&head, 4); push(&head, 15); push(&head, 85); printList(head); reverse(&head); printf("\n Reversed Linked list \n"); printList(head); getchar(); }

**Time Complexity:** O(n)

**Space Complexity:** O(1)

**Recursive Method:**

1) Divide the list in two parts - first node and rest of the linked list. 2) Call reverse for the rest of the linked list. 3) Link rest to first. 4) Fix head pointer

void recursiveReverse(struct node** head_ref) { struct node* first; struct node* rest; /* empty list */ if (*head_ref == NULL) return; /* suppose first = {1, 2, 3}, rest = {2, 3} */ first = *head_ref; rest = first->next; /* List has only one node */ if (rest == NULL) return; /* reverse the rest list and put the first element at the end */ recursiveReverse(&rest); first->next->next = first; /* tricky step -- see the diagram */ first->next = NULL; /* fix the head pointer */ *head_ref = rest; }

**Time Complexity:** O(n)

**Space Complexity:** O(1)

**References:**

http://cslibrary.stanford.edu/105/LinkedListProblems.pdf

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