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Count triplets in a sorted doubly linked list whose sum is equal to a given value x
  • Difficulty Level : Medium
  • Last Updated : 31 Dec, 2020

Given a sorted doubly linked list of distinct nodes(no two nodes have the same data) and a value x. Count triplets in the list that sum up to a given value x.

Examples:
 

Method 1 (Naive Approach): 
Using three nested loops generate all triplets and check whether elements in the triplet sum up to x or not.

C++




// C++ implementation to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
#include <bits/stdc++.h>
 
using namespace std;
 
// structure of node of doubly linked list
struct Node {
    int data;
    struct Node* next, *prev;
};
 
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
int countTriplets(struct Node* head, int x)
{
    struct Node* ptr1, *ptr2, *ptr3;
    int count = 0;
 
    // generate all possible triplets
    for (ptr1 = head; ptr1 != NULL; ptr1 = ptr1->next)
        for (ptr2 = ptr1->next; ptr2 != NULL; ptr2 = ptr2->next)
            for (ptr3 = ptr2->next; ptr3 != NULL; ptr3 = ptr3->next)
 
                // if elements in the current triplet sum up to 'x'
                if ((ptr1->data + ptr2->data + ptr3->data) == x)
 
                    // increment count
                    count++;
 
    // required count of triplets
    return count;
}
 
// A utility function to insert a new node at the
// beginning of doubly linked list
void insert(struct Node** head, int data)
{
    // allocate node
    struct Node* temp = new Node();
 
    // put in the data
    temp->data = data;
    temp->next = temp->prev = NULL;
 
    if ((*head) == NULL)
        (*head) = temp;
    else {
        temp->next = *head;
        (*head)->prev = temp;
        (*head) = temp;
    }
}
 
// Driver program to test above
int main()
{
    // start with an empty doubly linked list
    struct Node* head = NULL;
 
    // insert values in sorted order
    insert(&head, 9);
    insert(&head, 8);
    insert(&head, 6);
    insert(&head, 5);
    insert(&head, 4);
    insert(&head, 2);
    insert(&head, 1);
 
    int x = 17;
 
    cout << "Count = "
         << countTriplets(head, x);
    return 0;
}

Java




// Java implementation to count triplets
// in a sorted doubly linked list
// whose sum is equal to a given value 'x'
import java.io.*;
import java.util.*;
 
// Represents node of a doubly linked list
class Node
{
    int data;
    Node prev, next;
    Node(int val)
    {
        data = val;
        prev = null;
        next = null;
    }
}
 
class GFG
{
 
    // function to count triplets in
    // a sorted doubly linked list
    // whose sum is equal to a given value 'x'
    static int countTriplets(Node head, int x)
    {
            Node ptr1, ptr2, ptr3;
            int count = 0;
 
            // generate all possible triplets
            for (ptr1 = head; ptr1 != null; ptr1 = ptr1.next)
                for (ptr2 = ptr1.next; ptr2 != null; ptr2 = ptr2.next)
                    for (ptr3 = ptr2.next; ptr3 != null; ptr3 = ptr3.next)
 
                        // if elements in the current triplet sum up to 'x'
                        if ((ptr1.data + ptr2.data + ptr3.data) == x)
                             
                            // increment count
                            count++;
 
            // required count of triplets
            return count;
    }
 
    // A utility function to insert a new node at the
    // beginning of doubly linked list
    static Node insert(Node head, int val)
    {
            // allocate node
            Node temp = new Node(val);
 
            if (head == null)
                head = temp;
 
            else
            {
                temp.next = head;
                head.prev = temp;
                head = temp;
            }
         
            return head;
    }
 
    // Driver code
    public static void main(String args[])
    {
            // start with an empty doubly linked list
            Node head = null;
             
            // insert values in sorted order
            head = insert(head, 9);
            head = insert(head, 8);
            head = insert(head, 6);
            head = insert(head, 5);
            head = insert(head, 4);
            head = insert(head, 2);
            head = insert(head, 1);
 
            int x = 17;
            System.out.println("count = " + countTriplets(head, x));
    }
}
 
// This code is contributed by rachana soma

Python3




# Python3 implementation to count triplets
# in a sorted doubly linked list
# whose sum is equal to a given value 'x'
 
# structure of node of doubly linked list
class Node:
    def __init__(self):
        self.data = None
        self.prev = None
        self.next = None
 
# function to count triplets in a sorted doubly linked list
# whose sum is equal to a given value 'x'
def countTriplets( head, x):
 
    ptr1 = head
    ptr2 = None
    ptr3 = None
    count = 0
 
    # generate all possible triplets
    while (ptr1 != None ):
        ptr2 = ptr1.next
        while ( ptr2 != None ):
            ptr3 = ptr2.next
            while ( ptr3 != None ):
         
                # if elements in the current triplet sum up to 'x'
                if ((ptr1.data + ptr2.data + ptr3.data) == x):
 
                    # increment count
                    count = count + 1
                ptr3 = ptr3.next
            ptr2 = ptr2.next
        ptr1 = ptr1.next
 
    # required count of triplets
    return count
 
# A utility function to insert a new node at the
# beginning of doubly linked list
def insert(head, data):
 
    # allocate node
    temp = Node()
 
    # put in the data
    temp.data = data
    temp.next = temp.prev = None
 
    if ((head) == None):
        (head) = temp
    else :
        temp.next = head
        (head).prev = temp
        (head) = temp
    return head
     
# Driver code
 
# start with an empty doubly linked list
head = None
 
# insert values in sorted order
head = insert(head, 9)
head = insert(head, 8)
head = insert(head, 6)
head = insert(head, 5)
head = insert(head, 4)
head = insert(head, 2)
head = insert(head, 1)
 
x = 17
 
print( "Count = ", countTriplets(head, x))
 
# This code is contributed by Arnab Kundu

C#




// C# implementation to count triplets
// in a sorted doubly linked list
// whose sum is equal to a given value 'x'
using System;
 
// Represents node of a doubly linked list
public class Node
{
    public int data;
    public Node prev, next;
    public Node(int val)
    {
        data = val;
        prev = null;
        next = null;
    }
}
 
class GFG
{
 
    // function to count triplets in
    // a sorted doubly linked list
    // whose sum is equal to a given value 'x'
    static int countTriplets(Node head, int x)
    {
        Node ptr1, ptr2, ptr3;
        int count = 0;
 
        // generate all possible triplets
        for (ptr1 = head; ptr1 != null; ptr1 = ptr1.next)
            for (ptr2 = ptr1.next; ptr2 != null; ptr2 = ptr2.next)
                for (ptr3 = ptr2.next; ptr3 != null; ptr3 = ptr3.next)
 
                    // if elements in the current triplet sum up to 'x'
                    if ((ptr1.data + ptr2.data + ptr3.data) == x)
                         
                        // increment count
                        count++;
 
        // required count of triplets
        return count;
    }
 
    // A utility function to insert a new node at the
    // beginning of doubly linked list
    static Node insert(Node head, int val)
    {
        // allocate node
        Node temp = new Node(val);
 
        if (head == null)
            head = temp;
 
        else
        {
            temp.next = head;
            head.prev = temp;
            head = temp;
        }
     
        return head;
    }
 
    // Driver code
    public static void Main(String []args)
    {
            // start with an empty doubly linked list
            Node head = null;
             
            // insert values in sorted order
            head = insert(head, 9);
            head = insert(head, 8);
            head = insert(head, 6);
            head = insert(head, 5);
            head = insert(head, 4);
            head = insert(head, 2);
            head = insert(head, 1);
 
            int x = 17;
            Console.WriteLine("count = " + countTriplets(head, x));
    }
}
 
// This code is contributed by Arnab Kundu

Output: 



Count = 2

Time Complexity: O(n3
Auxiliary Space: O(1)

Method 2 (Hashing): 
Create a hash table with (key, value) tuples represented as (node data, node pointer) tuples. Traverse the doubly linked list and store each node’s data and its pointer pair(tuple) in the hash table. Now, generate each possible pair of nodes. For each pair of nodes, calculate the p_sum(sum of data in the two nodes) and check whether (x-p_sum) exists in the hash table or not. If it exists, then also verify that the two nodes in the pair are not same to the node associated with (x-p_sum) in the hash table and finally increment count. Return (count / 3) as each triplet is counted 3 times in the above process.

C++




// C++ implementation to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
#include <bits/stdc++.h>
 
using namespace std;
 
// structure of node of doubly linked list
struct Node {
    int data;
    struct Node* next, *prev;
};
 
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
int countTriplets(struct Node* head, int x)
{
    struct Node* ptr, *ptr1, *ptr2;
    int count = 0;
 
    // unordered_map 'um' implemented as hash table
    unordered_map<int, Node*> um;
 
    // insert the <node data, node pointer> tuple in 'um'
    for (ptr = head; ptr != NULL; ptr = ptr->next)
        um[ptr->data] = ptr;
 
    // generate all possible pairs
    for (ptr1 = head; ptr1 != NULL; ptr1 = ptr1->next)
        for (ptr2 = ptr1->next; ptr2 != NULL; ptr2 = ptr2->next) {
 
            // p_sum - sum of elements in the current pair
            int p_sum = ptr1->data + ptr2->data;
 
            // if 'x-p_sum' is present in 'um' and either of the two nodes
            // are not equal to the 'um[x-p_sum]' node
            if (um.find(x - p_sum) != um.end() && um[x - p_sum] != ptr1
                && um[x - p_sum] != ptr2)
 
                // increment count
                count++;
        }
 
    // required count of triplets
    // division by 3 as each triplet is counted 3 times
    return (count / 3);
}
 
// A utility function to insert a new node at the
// beginning of doubly linked list
void insert(struct Node** head, int data)
{
    // allocate node
    struct Node* temp = new Node();
 
    // put in the data
    temp->data = data;
    temp->next = temp->prev = NULL;
 
    if ((*head) == NULL)
        (*head) = temp;
    else {
        temp->next = *head;
        (*head)->prev = temp;
        (*head) = temp;
    }
}
 
// Driver program to test above
int main()
{
    // start with an empty doubly linked list
    struct Node* head = NULL;
 
    // insert values in sorted order
    insert(&head, 9);
    insert(&head, 8);
    insert(&head, 6);
    insert(&head, 5);
    insert(&head, 4);
    insert(&head, 2);
    insert(&head, 1);
 
    int x = 17;
 
    cout << "Count = "
         << countTriplets(head, x);
    return 0;
}

Java




// Java implementation to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
import java.util.*;
 
class GFG{
  
// structure of node of doubly linked list
static class Node {
    int data;
    Node next, prev;
    Node(int val)
    {
        data = val;
        prev = null;
        next = null;
    }
};
  
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
static int countTriplets(Node head, int x)
{
    Node ptr, ptr1, ptr2;
    int count = 0;
  
    // unordered_map 'um' implemented as hash table
    HashMap<Integer,Node> um = new HashMap<Integer,Node>();
  
    // insert the <node data, node pointer> tuple in 'um'
    for (ptr = head; ptr != null; ptr = ptr.next)
        um.put(ptr.data, ptr);
  
    // generate all possible pairs
    for (ptr1 = head; ptr1 != null; ptr1 = ptr1.next)
        for (ptr2 = ptr1.next; ptr2 != null; ptr2 = ptr2.next) {
  
            // p_sum - sum of elements in the current pair
            int p_sum = ptr1.data + ptr2.data;
  
            // if 'x-p_sum' is present in 'um' and either of the two nodes
            // are not equal to the 'um[x-p_sum]' node
            if (um.containsKey(x - p_sum) && um.get(x - p_sum) != ptr1
                && um.get(x - p_sum) != ptr2)
  
                // increment count
                count++;
        }
  
    // required count of triplets
    // division by 3 as each triplet is counted 3 times
    return (count / 3);
}
  
// A utility function to insert a new node at the
// beginning of doubly linked list
static Node insert(Node head, int val)
{
        // allocate node
        Node temp = new Node(val);
 
        if (head == null)
            head = temp;
 
        else
        {
            temp.next = head;
            head.prev = temp;
            head = temp;
        }
      
        return head;
}
  
// Driver program to test above
public static void main(String[] args)
{
    // start with an empty doubly linked list
    Node head = null;
  
    // insert values in sorted order
    head = insert(head, 9);
    head = insert(head, 8);
    head = insert(head, 6);
    head = insert(head, 5);
    head = insert(head, 4);
    head = insert(head, 2);
    head = insert(head, 1);
  
    int x = 17;
  
    System.out.print("Count = "
         + countTriplets(head, x));
}
}
 
// This code is contributed by Rajput-Ji

Python3




# Python3 implementation to count triplets in a sorted doubly linked list
# whose sum is equal to a given value 'x'
  
# structure of node of doubly linked list
class Node:
     
    def __init__(self, data):
         
        self.data=data
        self.next=None
        self.prev=None
 
  
# function to count triplets in a sorted doubly linked list
# whose sum is equal to a given value 'x'
def countTriplets(head, x):
 
    ptr2=head
    count = 0;
  
    # unordered_map 'um' implemented as hash table
    um = dict()
     
    ptr = head
    # insert the <node data, node pointer> tuple in 'um'
    while ptr!=None:
        um[ptr.data] = ptr;
        ptr = ptr.next
  
    # generate all possible pairs
    ptr1=head
     
    while ptr1!=None:
         
        ptr2 = ptr1.next
         
        while ptr2!=None:
  
            # p_sum - sum of elements in the current pair
            p_sum = ptr1.data + ptr2.data;
  
            # if 'x-p_sum' is present in 'um' and either of the two nodes
            # are not equal to the 'um[x-p_sum]' node
            if ((x-p_sum) in um) and um[x - p_sum] != ptr1 and um[x - p_sum] != ptr2:
  
                # increment count
                count+=1
            ptr2 = ptr2.next
        ptr1 = ptr1.next
         
  
    # required count of triplets
    # division by 3 as each triplet is counted 3 times
    return (count // 3);
 
  
# A utility function to insert a new node at the
# beginning of doubly linked list
def insert(head, data):
 
    # allocate node
    temp = Node(data);
  
    if ((head) == None):
        (head) = temp;
    else:
        temp.next = head;
        (head).prev = temp;
        (head) = temp;
    return head
     
  
# Driver program to test above
if __name__=='__main__':
     
    # start with an empty doubly linked list
    head = None;
  
    # insert values in sorted order
    head = insert(head, 9);
    head = insert(head, 8);
    head = insert(head, 6);
    head = insert(head, 5);
    head = insert(head, 4);
    head = insert(head, 2);
    head = insert( head, 1);
  
    x = 17;
  
    print("Count = "+ str(countTriplets(head, x)))
     
# This code is contributed by rutvik_56

C#




// C# implementation to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
using System;
using System.Collections.Generic;
 
class GFG
{
 
// structure of node of doubly linked list
class Node {
    public int data;
    public Node next, prev;
    public Node(int val)
    {
        data = val;
        prev = null;
        next = null;
    }
};
 
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
static int countTriplets(Node head, int x)
{
    Node ptr, ptr1, ptr2;
    int count = 0;
 
    // unordered_map 'um' implemented as hash table
    Dictionary<int,Node> um = new Dictionary<int,Node>();
 
    // insert the <node data, node pointer> tuple in 'um'
    for (ptr = head; ptr != null; ptr = ptr.next)
        if(um.ContainsKey(ptr.data))
            um[ptr.data] = ptr;
        else
            um.Add(ptr.data, ptr);
 
    // generate all possible pairs
    for (ptr1 = head; ptr1 != null; ptr1 = ptr1.next)
        for (ptr2 = ptr1.next; ptr2 != null; ptr2 = ptr2.next)
        {
 
            // p_sum - sum of elements in the current pair
            int p_sum = ptr1.data + ptr2.data;
 
            // if 'x-p_sum' is present in 'um' and either of the two nodes
            // are not equal to the 'um[x-p_sum]' node
            if (um.ContainsKey(x - p_sum) && um[x - p_sum] != ptr1
                && um[x - p_sum] != ptr2)
 
                // increment count
                count++;
        }
 
    // required count of triplets
    // division by 3 as each triplet is counted 3 times
    return (count / 3);
}
 
// A utility function to insert a new node at the
// beginning of doubly linked list
static Node insert(Node head, int val)
{
        // allocate node
        Node temp = new Node(val);
 
        if (head == null)
            head = temp;
 
        else
        {
            temp.next = head;
            head.prev = temp;
            head = temp;
        }
     
        return head;
}
 
// Driver code
public static void Main(String[] args)
{
    // start with an empty doubly linked list
    Node head = null;
 
    // insert values in sorted order
    head = insert(head, 9);
    head = insert(head, 8);
    head = insert(head, 6);
    head = insert(head, 5);
    head = insert(head, 4);
    head = insert(head, 2);
    head = insert(head, 1);
 
    int x = 17;
 
    Console.Write("Count = "
        + countTriplets(head, x));
}
}
 
// This code is contributed by PrinciRaj1992

Output: 

Count = 2

Time Complexity: O(n2
Auxiliary Space: O(n)

Method 3 Efficient Approach(Use of two pointers): 
Traverse the doubly linked list from left to right. For each current node during the traversal, initailze two pointers first = pointer to the node next to the current node and last = pointer to the last node of the list. Now, count pairs in the list from first to last pointer that sum up to value (x – current node’s data) (algorithm described in this post). Add this count to the total_count of triplets. Pointer to the last node can be found only once in the beginning.

C++




// C++ implementation to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
#include <bits/stdc++.h>
 
using namespace std;
 
// structure of node of doubly linked list
struct Node {
    int data;
    struct Node* next, *prev;
};
 
// function to count pairs whose sum equal to given 'value'
int countPairs(struct Node* first, struct Node* second, int value)
{
    int count = 0;
 
    // The loop terminates when either of two pointers
    // become NULL, or they cross each other (second->next
    // == first), or they become same (first == second)
    while (first != NULL && second != NULL &&
           first != second && second->next != first) {
 
        // pair found
        if ((first->data + second->data) == value) {
 
            // increment count
            count++;
 
            // move first in forward direction
            first = first->next;
 
            // move second in backward direction
            second = second->prev;
        }
 
        // if sum is greater than 'value'
        // move second in backward direction
        else if ((first->data + second->data) > value)
            second = second->prev;
 
        // else move first in forward direction
        else
            first = first->next;
    }
 
    // required count of pairs
    return count;
}
 
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
int countTriplets(struct Node* head, int x)
{
    // if list is empty
    if (head == NULL)
        return 0;
 
    struct Node* current, *first, *last;
    int count = 0;
 
    // get pointer to the last node of
    // the doubly linked list
    last = head;
    while (last->next != NULL)
        last = last->next;
 
    // traversing the doubly linked list
    for (current = head; current != NULL; current = current->next) {
 
        // for each current node
        first = current->next;
 
        // count pairs with sum(x - current->data) in the range
        // first to last and add it to the 'count' of triplets
        count += countPairs(first, last, x - current->data);
    }
 
    // required count of triplets
    return count;
}
 
// A utility function to insert a new node at the
// beginning of doubly linked list
void insert(struct Node** head, int data)
{
    // allocate node
    struct Node* temp = new Node();
 
    // put in the data
    temp->data = data;
    temp->next = temp->prev = NULL;
 
    if ((*head) == NULL)
        (*head) = temp;
    else {
        temp->next = *head;
        (*head)->prev = temp;
        (*head) = temp;
    }
}
 
// Driver program to test above
int main()
{
    // start with an empty doubly linked list
    struct Node* head = NULL;
 
    // insert values in sorted order
    insert(&head, 9);
    insert(&head, 8);
    insert(&head, 6);
    insert(&head, 5);
    insert(&head, 4);
    insert(&head, 2);
    insert(&head, 1);
 
    int x = 17;
 
    cout << "Count = "
         << countTriplets(head, x);
    return 0;
}

Java




// Java implementation to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
import java.util.*;
 
class GFG{
  
// structure of node of doubly linked list
static class Node {
    int data;
    Node next, prev;
};
  
// function to count pairs whose sum equal to given 'value'
static int countPairs(Node first, Node second, int value)
{
    int count = 0;
  
    // The loop terminates when either of two pointers
    // become null, or they cross each other (second.next
    // == first), or they become same (first == second)
    while (first != null && second != null &&
           first != second && second.next != first) {
  
        // pair found
        if ((first.data + second.data) == value) {
  
            // increment count
            count++;
  
            // move first in forward direction
            first = first.next;
  
            // move second in backward direction
            second = second.prev;
        }
  
        // if sum is greater than 'value'
        // move second in backward direction
        else if ((first.data + second.data) > value)
            second = second.prev;
  
        // else move first in forward direction
        else
            first = first.next;
    }
  
    // required count of pairs
    return count;
}
  
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
static int countTriplets(Node head, int x)
{
    // if list is empty
    if (head == null)
        return 0;
  
    Node current, first, last;
    int count = 0;
  
    // get pointer to the last node of
    // the doubly linked list
    last = head;
    while (last.next != null)
        last = last.next;
  
    // traversing the doubly linked list
    for (current = head; current != null; current = current.next) {
  
        // for each current node
        first = current.next;
  
        // count pairs with sum(x - current.data) in the range
        // first to last and add it to the 'count' of triplets
        count += countPairs(first, last, x - current.data);
    }
  
    // required count of triplets
    return count;
}
  
// A utility function to insert a new node at the
// beginning of doubly linked list
static Node insert(Node head, int data)
{
    // allocate node
    Node temp = new Node();
  
    // put in the data
    temp.data = data;
    temp.next = temp.prev = null;
  
    if ((head) == null)
        (head) = temp;
    else {
        temp.next = head;
        (head).prev = temp;
        (head) = temp;
    }
    return head;
}
  
// Driver program to test above
public static void main(String[] args)
{
    // start with an empty doubly linked list
    Node head = null;
  
    // insert values in sorted order
    head = insert(head, 9);
    head = insert(head, 8);
    head = insert(head, 6);
    head = insert(head, 5);
    head = insert(head, 4);
    head = insert(head, 2);
    head = insert(head, 1);
  
    int x = 17;
  
    System.out.print("Count = "
         + countTriplets(head, x));
}
}
 
// This code is contributed by 29AjayKumar

Python3




# Python3 implementation to count triplets
# in a sorted doubly linked list whose sum
# is equal to a given value 'x'
 
# Structure of node of doubly linked list
class Node:
     
    def __init__(self, x):
         
        self.data = x
        self.next = None
        self.prev = None
 
# Function to count pairs whose sum
# equal to given 'value'
def countPairs(first, second, value):
     
    count = 0
 
    # The loop terminates when either of two pointers
    # become None, or they cross each other (second.next
    # == first), or they become same (first == second)
    while (first != None and second != None and
           first != second and second.next != first):
 
        # Pair found
        if ((first.data + second.data) == value):
             
            # Increment count
            count += 1
 
            # Move first in forward direction
            first = first.next
 
            # Move second in backward direction
            second = second.prev
 
        # If sum is greater than 'value'
        # move second in backward direction
        elif ((first.data + second.data) > value):
            second = second.prev
 
        # Else move first in forward direction
        else:
            first = first.next
 
    # Required count of pairs
    return count
 
# Function to count triplets in a sorted
# doubly linked list whose sum is equal
# to a given value 'x'
def countTriplets(head, x):
     
    # If list is empty
    if (head == None):
        return 0
 
    current, first, last = head, None, None
    count = 0
 
    # Get pointer to the last node of
    # the doubly linked list
    last = head
     
    while (last.next != None):
        last = last.next
 
    # Traversing the doubly linked list
    while current != None:
 
        # For each current node
        first = current.next
 
        # count pairs with sum(x - current.data) in
        # the range first to last and add it to the
        # 'count' of triplets
        count, current = count + countPairs(
            first, last, x - current.data), current.next
 
    # Required count of triplets
    return count
 
# A utility function to insert a new node
# at the beginning of doubly linked list
def insert(head, data):
     
    # Allocate node
    temp = Node(data)
 
    # Put in the data
    # temp.next = temp.prev = None
    if (head == None):
        head = temp
    else:
        temp.next = head
        head.prev = temp
        head = temp
         
    return head
 
# Driver code
if __name__ == '__main__':
     
    # Start with an empty doubly linked list
    head = None
 
    # Insert values in sorted order
    head = insert(head, 9)
    head = insert(head, 8)
    head = insert(head, 6)
    head = insert(head, 5)
    head = insert(head, 4)
    head = insert(head, 2)
    head = insert(head, 1)
 
    x = 17
 
    print("Count = ", countTriplets(head, x))
     
# This code is contributed by mohit kumar 29

C#




// C# implementation to count triplets
// in a sorted doubly linked list
// whose sum is equal to a given value 'x'
using System;
 
class GFG
{
 
// structure of node of doubly linked list
class Node
{
    public int data;
    public Node next, prev;
};
 
// function to count pairs whose sum equal to given 'value'
static int countPairs(Node first, Node second, int value)
{
    int count = 0;
 
    // The loop terminates when either of two pointers
    // become null, or they cross each other (second.next
    // == first), or they become same (first == second)
    while (first != null && second != null &&
        first != second && second.next != first) {
 
        // pair found
        if ((first.data + second.data) == value) {
 
            // increment count
            count++;
 
            // move first in forward direction
            first = first.next;
 
            // move second in backward direction
            second = second.prev;
        }
 
        // if sum is greater than 'value'
        // move second in backward direction
        else if ((first.data + second.data) > value)
            second = second.prev;
 
        // else move first in forward direction
        else
            first = first.next;
    }
 
    // required count of pairs
    return count;
}
 
// function to count triplets in a sorted doubly linked list
// whose sum is equal to a given value 'x'
static int countTriplets(Node head, int x)
{
    // if list is empty
    if (head == null)
        return 0;
 
    Node current, first, last;
    int count = 0;
 
    // get pointer to the last node of
    // the doubly linked list
    last = head;
    while (last.next != null)
        last = last.next;
 
    // traversing the doubly linked list
    for (current = head; current != null; current = current.next) {
 
        // for each current node
        first = current.next;
 
        // count pairs with sum(x - current.data) in the range
        // first to last and add it to the 'count' of triplets
        count += countPairs(first, last, x - current.data);
    }
 
    // required count of triplets
    return count;
}
 
// A utility function to insert a new node at the
// beginning of doubly linked list
static Node insert(Node head, int data)
{
    // allocate node
    Node temp = new Node();
 
    // put in the data
    temp.data = data;
    temp.next = temp.prev = null;
 
    if ((head) == null)
        (head) = temp;
    else {
        temp.next = head;
        (head).prev = temp;
        (head) = temp;
    }
    return head;
}
 
// Driver program to test above
public static void Main(String[] args)
{
    // start with an empty doubly linked list
    Node head = null;
 
    // insert values in sorted order
    head = insert(head, 9);
    head = insert(head, 8);
    head = insert(head, 6);
    head = insert(head, 5);
    head = insert(head, 4);
    head = insert(head, 2);
    head = insert(head, 1);
 
    int x = 17;
 
    Console.Write("Count = "
        + countTriplets(head, x));
}
}
 
// This code is contributed by 29AjayKumar

Output: 

Count = 2

Time Complexity: O(n2
Auxiliary Space: O(1)

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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