C Exercises: Pair in a linked list whose sum is equal to a given value

Last update on August 04 2025 13:01:31 (UTC/GMT +8 hours)

38. Pair Sum Search Challenges

Write a C program to find a pair in a singly linked list whose sum is equal to a given value.

Sample Solution:

C Code:

#include<stdio.h>
#include<stdlib.h>

// Structure defining a node in a singly linked list
struct Node {
    int data;           // Data stored in the node
    struct Node* next;  // Pointer to the next node
};

// Function to create a new node with given data
struct Node* newNode(int data) {
    struct Node* node = (struct Node*) malloc(sizeof(struct Node)); // Allocate memory for a new node
    node->data = data;  // Assign data to the new node
    node->next = NULL;  // Initialize the next pointer as NULL
    return node;  // Return the newly created node
}

// Function to find pairs in a linked list that sum up to a given value
void pairSum(struct Node *head, int sum) {
    struct Node *first = head;  // Pointer for the first node
    struct Node *second = NULL; // Pointer for the second node
    int flag = 0; // Flag to indicate if pairs are found

    // Loop through the list
    while (first != NULL && first->next != NULL) {
        second = first->next; // Initialize second pointer to the node after the first

        // Loop to compare each node's data with other nodes' data in the list
        while (second != NULL) {
            if ((first->data + second->data) == sum) { // Check if the sum matches the given value
                printf("(%d,%d) ", first->data, second->data); // Print the pair
                flag = 1; // Set the flag to indicate pair(s) found
            }
            second = second->next; // Move the second pointer to the next node
        }

        first = first->next; // Move the first pointer to the next node
    }

    if(flag==0)
        printf("Pair not found.\n"); // If no pairs found, print message
}

// Function to print elements of a linked list
void displayList(struct Node* head) {
    while (head) {
        printf("%d ", head->data);  // Print the data of the current node
        head = head->next;  // Move to the next node
    }
    printf("\n");
}

// Main function to demonstrate finding pairs in a linked list that sum up to a given value
int main() {
    struct Node* head = newNode(1); // Create the linked list with some initial nodes
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
    head->next->next->next->next->next = newNode(6);
    head->next->next->next->next->next ->next = newNode(7);
    printf("Original singly linked list:\n");
    displayList(head); // Display the original list

    int s = 4; // The sum value to find pairs
    printf("\nFind a pair whose sum is equal to 4:\n");
    pairSum(head, s); // Find pairs that sum up to 4

    // Test with different sum values
    s = 11;
    printf("\n\nFind a pair whose sum is equal to 11:\n");
    pairSum(head, s);
    s = 5;
    printf("\n\nFind a pair whose sum is equal to 5:\n");
    pairSum(head, s);
    s = 14;
    printf("\n\nFind a pair whose sum is equal to 14:\n");
    pairSum(head, s);

    return 0;
}

Sample Output:

Original singly linked list:
1 2 3 4 5 6 7 

Find a pair whose sum is equal to 4:
(1,3) 

Find a pair whose sum is equal to 11:
(4,7) (5,6) 

Find a pair whose sum is equal to 5:
(1,4) (2,3) 

Find a pair whose sum is equal to 14:
Pair not found.

Flowchart :

For more Practice: Solve these Related Problems:

Write a C program to find all unique pairs in a singly linked list whose sum equals a given target without using extra space.
Write a C program to find a pair whose product equals a given value in a singly linked list.
Write a C program to sort a linked list and then use a two-pointer approach to find pairs that sum to a target value.
Write a C program to identify and print multiple pairs from a linked list that add up to a given sum, handling duplicates appropriately.

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