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Find triplet sum closest to X in a sorted Doubly Linked List (DLL)

  • Last Updated : 14 Jul, 2021

Given a sorted doubly linked list of N nodes and an integer X, the task is to find the sum of three nodes in the list which is closest to X


Input: DLL: -8 ↔ 2 ↔ 3 ↔ 4 ↔ 5, X = 1
Output: 1
Explanation: The required three integers {-8, 4, 5} whose sum is 1 and is closest to 1. 

Input: DLL: 1 ↔ 2 ↔ 3 ↔ 4, X = 3 
Output: 6
Explanation: The required three integers are {1, 2, 3} whose sum is 6 and is closest to X = 3. 

Naive Approach: The simplest approach to solve the given problem is to generate all possible triplets using three nested loops and then choose the triplet which has the sum closest to X and print the sum of the triplet.

Time Complexity: O(N3)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, the idea is to use 3 pointer technique. Follow the steps below to solve this problem:

  • Initialize 4 variables, first and second pointing to the head node of the doubly linked list, i.e., first = head, second = head, and tail and third both initialized to the last node of the doubly linked list.
  • Initialize a variable diff, initialized as INT_MAX, which stores the closest sum to X.
  • Iterate while the first node is not NULL and perform the following steps:
    • Initialize second to the next of first, i.e., second = first→next and third = tail(the last node of doubly linked list).
    • Iterate while second and third are not NULL and third is not equal to the second.
      • Initialize a variable say, sum as (first→data + second→data + third→data).
      • If the absolute value of X – sum is less than the absolute value of X diff, then update the value of diff as sum.
      • If sum is less than X, then increment second pointer, i.e., second = second→next.
      • Otherwise, decrement third, i.e., third = third→prev.
    • Move the first pointer to the next pointer, i.e., first = first→next.
  • After completing the above steps, print the value of diff as the result.

Below is the implementation of the above approach:


// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
// Doubly linked list node
struct Node {
    int data;
    struct Node *next, *prev;
// 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();
    // Fill the data value in it
    temp->data = data;
    temp->next = temp->prev = NULL;
    // If head is NULL change
    // head to temp
    if ((*head) == NULL) {
        (*head) = temp;
    // Insert the node before head
    else {
        temp->next = *head;
        (*head)->prev = temp;
        (*head) = temp;
// Function to find sum of triplet
// closest to X
void closestTripletSum(struct Node* head, int X)
    // Points to the first node
    // of the triplet
    struct Node* first = head;
    // Points to the second node
    // of the triplet
    struct Node* second = head->next;
    // Stores the last node of
    // doubly linked list
    struct Node* tail = head;
    // Traverse till end of the list
    while (tail->next != NULL) {
        tail = tail->next;
    // Stores the sum closest to X
    int diff = INT_MAX;
    // Points to the third node
    // of the triplet
    struct Node* third = tail;
    // Iterate till the end of the list
    while (first != NULL) {
        second = first->next;
        third = tail;
        while (second != NULL && third != NULL
               && third != second) {
            int sum = (first->data + second->data
                       + third->data);
            // Check if the current sum
            // is closest to X
            if (abs(X - sum) < abs(X - diff)) {
                // Update the value of diff
                diff = sum;
            // Check if sum is less than X
            if (sum < X) {
                // Increment the second
                // pointer
                second = second->next;
            else {
                // Decrement the third
                // pointer
                third = third->prev;
        // Move the first pointer
        // ahead
        first = first->next;
    // Print the closest sum
    cout << diff;
// Driver Code
int main()
    // Given Input
    struct Node* head = NULL;
    insert(&head, 4);
    insert(&head, 3);
    insert(&head, 2);
    insert(&head, 1);
    int X = 3;
    // Function Call
    closestTripletSum(head, X);
    return 0;

Time Complexity: O(N2)
Auxiliary Space: O(1)

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