Three Sum Problem All Unique Triplets in DSA C - Build from Scratch

Three Sum Problem All Unique Triplets

📖 Scenario: You are working on a financial app that analyzes daily stock price changes. You want to find all unique sets of three days where the total change sums to zero, indicating a balanced period.

🎯 Goal: Build a program that finds all unique triplets in an integer array that sum to zero.

📋 What You'll Learn

Create an integer array called nums with the exact values: -1, 0, 1, 2, -1, -4

Create an integer variable called numsSize and set it to the size of nums

Implement a function threeSum that takes nums and numsSize and prints all unique triplets that sum to zero

Print each triplet in the format: [a, b, c] on its own line

💡 Why This Matters

🌍 Real World

Finding balanced periods in stock price changes or financial data helps analysts detect stability or reversal points.

💼 Career

This problem is common in technical interviews and helps develop skills in sorting, two-pointer technique, and handling duplicates.

1

Create the input array

Create an integer array called nums with these exact values: -1, 0, 1, 2, -1, -4.

DSA C

# Your code here

Hint

Use curly braces to list the values inside the array.

2

Set the array size

Create an integer variable called numsSize and set it to the size of the nums array (6).

DSA C

int nums[] = {-1, 0, 1, 2, -1, -4};
# Your code here

Hint

Count the number of elements in nums and assign it to numsSize.

3

Implement the threeSum function

Implement a function called threeSum that takes nums and numsSize as parameters and prints all unique triplets that sum to zero. Use sorting and two-pointer technique inside the function.

DSA C

int nums[] = {-1, 0, 1, 2, -1, -4};
int numsSize = 6;

// Your code here

Hint

Sort the array first. Then use a for loop with two pointers to find triplets summing to zero. Skip duplicates.

4

Call threeSum and print results

Call the threeSum function with nums and numsSize to print all unique triplets that sum to zero.

DSA C

#include 
#include 

int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

void threeSum(int* nums, int numsSize) {
    qsort(nums, numsSize, sizeof(int), compare);
    for (int i = 0; i  0 && nums[i] == nums[i - 1]) continue;
        int left = i + 1;
        int right = numsSize - 1;
        while (left < right) {
            int sum = nums[i] + nums[left] + nums[right];
            if (sum == 0) {
                printf("[%d, %d, %d]\n", nums[i], nums[left], nums[right]);
                while (left < right && nums[left] == nums[left + 1]) left++;
                while (left < right && nums[right] == nums[right - 1]) right--;
                left++;
                right--;
            } else if (sum < 0) {
                left++;
            } else {
                right--;
            }
        }
    }
}

int main() {
    int nums[] = {-1, 0, 1, 2, -1, -4};
    int numsSize = 6;
    // Your code here
}

Hint

Call threeSum(nums, numsSize); inside main to print the triplets.