Check if a Linked List is Pairwise Sorted

Last Updated : 22 Dec, 2022

Given a linked list. The task is to check if the linked list is pairwise sorted or not.

A Linked List is considered pairwise sorted if each successive pair of numbers is in sorted (non-decreasing) order. In the case of odd number of nodes, the last node is ignored and the result is based on the remaining even number of nodes.

Examples:

Input: List =  10 -> 15 -> 9 -> 9 -> 1 -> 5 Output: YES  Input: List = 10 -> 15 -> 8 -> 9 -> 10 -> 5 Output: NO

Approach: The idea is to traverse the linked list from left to right. Compare nodes pairwise, if any pair violates property, return false. If no pair violates property, return True.

Below is the implementation of above approach:

C++

// C++ program to check if linked list is // pairwise sorted #include <iostream>  using namespace std;  // A linked list node struct Node {     int data;     struct Node* next; };  // Function to check if linked list is // pairwise sorted bool isPairWiseSorted(struct Node* head) {     bool flag = true;      struct Node* temp = head;      // Traverse further only if there     // are at-least two nodes left     while (temp != NULL && temp->next != NULL) {         if (temp->data > temp->next->data) {             flag = false;             break;         }          temp = temp->next->next;     }      return flag; }  // Function to add a node at the // beginning of Linked List 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 of the new node */     new_node->next = (*head_ref);      /* move the head to point to the new node */     (*head_ref) = new_node; }  // Driver program to test above function int main() {     struct Node* start = NULL;      /* The constructed linked list is:      10->15->9->9->1->5 */     push(&start, 5);     push(&start, 1);     push(&start, 9);     push(&start, 9);     push(&start, 15);     push(&start, 10);      if (isPairWiseSorted(start))         cout << "YES";     else         cout << "NO";      return 0; }

Java

// Java program to check if linked list is  // pairwise sorted  class GFG {   // A linked list node  static class Node {      int data;      Node next;  };  static Node start;  // Function to check if linked list is  // pairwise sorted  static boolean isPairWiseSorted(Node head)  {      boolean flag = true;       Node temp = head;       // Traverse further only if there      // are at-least two nodes left      while (temp != null && temp.next != null)      {          if (temp.data > temp.next.data)          {              flag = false;              break;          }           temp = temp.next.next;      }      return flag;  }   // Function to add a node at the  // beginning of Linked List  static void push(Node head_ref, int new_data)  {           /* allocate node */     Node new_node = new Node();       /* put in the data */     new_node.data = new_data;       /* link the old list of the new node */     new_node.next = head_ref;       /* move the head to point to the new node */     head_ref = new_node;          start = head_ref; }   // Driver Code public static void main(String[] args)  {     start = null;       /* The constructed linked list is:      10.15.9.9.1.5 */     push(start, 5);      push(start, 1);      push(start, 9);      push(start, 9);      push(start, 15);      push(start, 10);       if (isPairWiseSorted(start))          System.out.println("YES");     else         System.out.println("NO");     } }  // This code is contributed by PrinciRaj1992

Python

# Python program to check if linked list is  # pairwise sorted   # Linked List node  class Node:      def __init__(self, data):          self.data = data          self.next = None          start = None  # Function to check if linked list is  # pairwise sorted  def isPairWiseSorted(head) :      flag = True      temp = head       # Traverse further only if there      # are at-least two nodes left      while (temp != None and temp.next != None) :              if (temp.data > temp.next.data) :             flag = False             break                  temp = temp.next.next          return flag   # Function to add a node at the  # beginning of Linked List  def push(head_ref, new_data) :      global start     # allocate node      new_node = Node(0)       # put in the data      new_node.data = new_data       # link the old list of the new node      new_node.next = head_ref       # move the head to point to the new node      head_ref = new_node      start = head_ref  # Driver Code start = None  # The constructed linked list is:  # 10.15.9.9.1.5  push(start, 5)  push(start, 1)  push(start, 9)  push(start, 9)  push(start, 15)  push(start, 10)   if (isPairWiseSorted(start)) :     print("YES") else:     print("NO")      # This code is contributed by Arnab Kundu

// C# program to check if linked list is  // pairwise sorted  using System; class GFG {   // A linked list node  public class Node {      public int data;      public Node next;  };  static Node start;  // Function to check if linked list is  // pairwise sorted  static Boolean isPairWiseSorted(Node head)  {      Boolean flag = true;       Node temp = head;       // Traverse further only if there      // are at-least two nodes left      while (temp != null && temp.next != null)      {          if (temp.data > temp.next.data)          {              flag = false;              break;          }           temp = temp.next.next;      }      return flag;  }   // Function to add a node at the  // beginning of Linked List  static void push(Node head_ref, int new_data)  {           /* allocate node */     Node new_node = new Node();       /* put in the data */     new_node.data = new_data;       /* link the old list of the new node */     new_node.next = head_ref;       /* move the head to point to the new node */     head_ref = new_node;      start = head_ref; }   // Driver Code public static void Main(String[] args)  {     start = null;       /* The constructed linked list is:      10.15.9.9.1.5 */     push(start, 5);      push(start, 1);      push(start, 9);      push(start, 9);      push(start, 15);      push(start, 10);       if (isPairWiseSorted(start))          Console.WriteLine("YES");     else         Console.WriteLine("NO");     } }  // This code is contributed by Rajput-Jiv

JavaScript

<script>        // JavaScript program to check if linked list is       // pairwise sorted       // A linked list node       class Node {         constructor() {           this.data = 0;           this.next = null;         }       }       var start = null;        // Function to check if linked list is       // pairwise sorted       function isPairWiseSorted(head) {         var flag = true;          var temp = head;          // Traverse further only if there         // are at-least two nodes left         while (temp != null && temp.next != null) {           if (temp.data > temp.next.data) {             flag = false;             break;           }            temp = temp.next.next;         }         return flag;       }        // Function to add a node at the       // beginning of Linked List       function push(head_ref, new_data) {         /* allocate node */         var new_node = new Node();          /* put in the data */         new_node.data = new_data;          /* link the old list of the new node */         new_node.next = head_ref;          /* move the head to point to the new node */         head_ref = new_node;         start = head_ref;       }        // Driver Code       start = null;        /* The constructed linked list is:     10.15.9.9.1.5 */       push(start, 5);       push(start, 1);       push(start, 9);       push(start, 9);       push(start, 15);       push(start, 10);        if (isPairWiseSorted(start)) document.write("YES");       else document.write("NO");        </script>

Output

YES

Complexity Analysis:

Time Complexity: O(n)
Auxiliary Space: O(1)

Check if a linked list is Circular Linked List

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Check if a Linked List is Pairwise Sorted

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