GATE 2022

Question 15

Consider the problem of reversing a singly linked list. To take an example, given the linked list below,

Which one of the following statements is TRUE about the time complexity of algorithms that solve the above problem in O (1) space?

A
B
C
D	It is not possible to reverse a singly linked list in O (1) space.

Data-StructuresLinked-ListVideo-Explanation

Question 15 Explanation:

/* Link list node */
struct Node {
int data;
struct Node* next;
{
this->data = data;
next = NULL;
}
};
struct LinkedList {
Node* head;
LinkedList() { head = NULL; }
/* Function to reverse the linked list */
void reverse()
{
// Initialize current, previous and
// next pointers
Node* current = head;
Node *prev = NULL, *next = NULL;
while (current != NULL) {
// Store next
next = current->next;
// Reverse current node’s pointer
current->next = prev;
// Move pointers one position ahead.
prev = current;
current = next;
}
head = prev;
}
For the above algorithm :
Time Complexity: O(n)
Space Complexity: O(1)

Correct Answer: A

Question 15 Explanation:

/* Link list node */
struct Node {
int data;
struct Node* next;
{
this->data = data;
next = NULL;
}
};
struct LinkedList {
Node* head;
LinkedList() { head = NULL; }
/* Function to reverse the linked list */
void reverse()
{
// Initialize current, previous and
// next pointers
Node* current = head;
Node *prev = NULL, *next = NULL;
while (current != NULL) {
// Store next
next = current->next;
// Reverse current node’s pointer
current->next = prev;
// Move pointers one position ahead.
prev = current;
current = next;
}
head = prev;
}
For the above algorithm :
Time Complexity: O(n)
Space Complexity: O(1)

GATE 2022

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GATE 2022

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GATE 2022