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 price change sums to zero. This helps identify balanced periods where gains and losses cancel out.

🎯 Goal: Build a program that finds all unique triplets in an integer array that sum to zero. The program should print each triplet as a list of three numbers.

📋 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 n that stores the length of nums

Implement the three sum logic using nested loops and sorting to find all unique triplets that sum to zero

Store the unique triplets in a list of lists called result

Print each triplet in result on a separate line in the format: [a, b, c]

💡 Why This Matters

🌍 Real World

Finding balanced triplets in financial data helps identify periods where gains and losses offset each other, useful for risk analysis.

💼 Career

This problem is common in coding interviews and tests your ability to use sorting, loops, and two-pointer techniques to solve array problems efficiently.

1

Create the input array

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

DSA C#

# Create the integer array nums with the given values
// Your code here

Hint

Use int[] nums = new int[] { -1, 0, 1, 2, -1, -4 }; to create the array.

2

Create a variable for the array length

Create an integer variable called n that stores the length of the nums array

DSA C#

int[] nums = new int[] { -1, 0, 1, 2, -1, -4 };
// Create an integer variable n to store the length of nums
// Your code here

Hint

Use int n = nums.Length; to get the length of the array.

3

Implement the three sum logic

Sort the nums array. Create a list of lists of integers called result to store unique triplets. Use a for loop with variable i from 0 to n - 3. Inside, use two pointers left and right to find triplets that sum to zero. Skip duplicates for i, left, and right. Add each unique triplet as a list to result.

DSA C#

int[] nums = new int[] { -1, 0, 1, 2, -1, -4 };
int n = nums.Length;
// Sort nums and implement the three sum logic to find unique triplets
// Your code here

Hint

Sort the array first. Use a for loop for i. Use two pointers left and right to find sums. Skip duplicates to avoid repeated triplets.

4

Print all unique triplets

Use a foreach loop to iterate over each triplet in result. Print each triplet in the format: [a, b, c] where a, b, and c are the three numbers in the triplet.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        int[] nums = new int[] { -1, 0, 1, 2, -1, -4 };
        int n = nums.Length;
        Array.Sort(nums);
        List> result = new List>();

        for (int i = 0; i  0 && nums[i] == nums[i - 1])
                continue;

            int left = i + 1;
            int right = n - 1;

            while (left  { 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--;
                }
            }
        }

        // Print each triplet in result
        // Your code here
    }
}

Hint

Use a foreach loop to print each triplet in result. Use Console.WriteLine with string interpolation to format the output.

Practice

1. What is the main goal of the Three Sum Problem All Unique Triplets in an integer array?

easy

A. Find all unique triplets that sum to zero

B. Find the maximum sum of any three numbers

C. Sort the array in ascending order

D. Count the number of pairs that sum to zero

Solution

1. Step 1: Understand the problem statement

   The problem asks to find all unique groups of three numbers (triplets) in the array whose sum equals zero.

2. Step 2: Clarify the output requirement

   We must return all such triplets without duplicates, not just count or sort.

3. Final Answer:

   Find all unique triplets that sum to zero -> Option A

4. Quick Check:

   Goal = unique triplets summing to zero [OK]

Hint: Look for three numbers adding to zero without duplicates [OK]

Common Mistakes:

• Confusing triplets with pairs
• Trying to find max sum instead of zero sum
• Ignoring uniqueness of triplets

2. Which of the following C# code snippets correctly initializes an integer array named nums for the Three Sum problem?

easy

A. int[] nums = (-1, 0, 1, 2, -1, -4);

B. int nums = { -1, 0, 1, 2, -1, -4 };

C. int[] nums = new int[] { -1, 0, 1, 2, -1, -4 };

D. int[] nums = new List<int> { -1, 0, 1, 2, -1, -4 };

Solution

1. Step 1: Check array initialization syntax in C#

   Correct syntax for array initialization is: int[] nums = new int[] { ... };

2. Step 2: Validate each option

   int[] nums = new int[] { -1, 0, 1, 2, -1, -4 }; uses correct syntax. int nums = { -1, 0, 1, 2, -1, -4 }; misses array type. int[] nums = (-1, 0, 1, 2, -1, -4); uses parentheses instead of braces. int[] nums = new List<int> { -1, 0, 1, 2, -1, -4 }; tries to assign a List to an array variable.

3. Final Answer:

   int[] nums = new int[] { -1, 0, 1, 2, -1, -4 }; -> Option C

4. Quick Check:

   Array init syntax = int[] nums = new int[] { -1, 0, 1, 2, -1, -4 }; [OK]

Hint: Use new int[] { ... } to initialize arrays in C# [OK]

Common Mistakes:

• Using parentheses instead of braces
• Assigning List to array variable
• Missing type declaration

3. What is the output of the following C# code snippet that finds all unique triplets summing to zero?

int[] nums = { -1, 0, 1, 2, -1, -4 };
var result = ThreeSum(nums);
foreach(var triplet in result) {
    Console.WriteLine(string.Join(", ", triplet));
}

// Assume ThreeSum returns List of unique triplets

medium

A. [0, 1, 2]
[-1, -1, -4]

B. [-1, 0, 1]
[-1, -1, 2]

C. [-4, 2, 2]
[-1, 0, 1]

D. [-1, 1, 2]
[0, 0, 0]

Solution

1. Step 1: Sort and find triplets summing to zero

   Sorted array: [-4, -1, -1, 0, 1, 2]. Triplets summing to zero are [-1, -1, 2] and [-1, 0, 1].

2. Step 2: Match output with options

   [-1, 0, 1] [-1, -1, 2] lists these two triplets correctly. Others contain invalid or impossible triplets.

3. Final Answer:

   [-1, 0, 1] [-1, -1, 2] -> Option B

4. Quick Check:

   Unique triplets = [-1, 0, 1] [-1, -1, 2] [OK]

Hint: Sort array first, then find triplets with two-pointer [OK]

Common Mistakes:

• Not sorting before searching
• Including duplicate triplets
• Wrong triplet sums

4. Identify the bug in this C# code snippet for the Three Sum problem:

public IList ThreeSum(int[] nums) {
    var result = new List();
    Array.Sort(nums);
    for (int i = 0; i < nums.Length - 2; i++) {
        int left = i + 1, right = nums.Length - 1;
        while (left < right) {
            int sum = nums[i] + nums[left] + nums[right];
            if (sum == 0) {
                result.Add(new List<int> { nums[i], nums[left], nums[right] });
                left++;
                right--;
            } else if (sum < 0) {
                left++;
            } else {
                right--;
            }
        }
    }
    return result;
}

medium

A. No check to skip duplicate values for i, left, and right

B. Array is not sorted before processing

C. Incorrect sum calculation formula

D. Using List instead of Array for result

Solution

1. Step 1: Analyze duplicate handling in loops

   The code does not skip duplicate values for i, left, or right, which can cause duplicate triplets in the result.

2. Step 2: Verify other parts

   Array is sorted, sum calculation is correct, and List usage is valid. So only duplicate skipping is missing.

3. Final Answer:

   No check to skip duplicate values for i, left, and right -> Option A

4. Quick Check:

   Missing duplicate skip = No check to skip duplicate values for i, left, and right [OK]

Hint: Always skip duplicates after finding a valid triplet [OK]

Common Mistakes:

• Not skipping duplicates causing repeated triplets
• Forgetting to sort array
• Wrong sum calculation

5. Given the input array nums = [-2, 0, 1, 2, -1, -1, 2], which of the following lists contains all unique triplets that sum to zero?

hard

A. [[-2, 0, 1], [-1, -1, 1], [0, 1, 2]]

B. [[-2, 1, 1], [-1, 0, 1], [0, 0, 0]]

C. [[-2, -1, 3], [-1, 1, 0], [1, 1, -2]]

D. [[-2, 0, 2], [-1, -1, 2], [-1, 0, 1]]

Solution

1. Step 1: Sort the array and find triplets

   Sorted nums: [-2, -1, -1, 0, 1, 2, 2]. Valid unique triplets summing to zero are [-2, 0, 2], [-1, -1, 2], and [-1, 0, 1].

2. Step 2: Compare with options

   [[-2, 0, 2], [-1, -1, 2], [-1, 0, 1]] matches these triplets exactly. Others contain invalid or duplicate triplets.

3. Final Answer:

   [[-2, 0, 2], [-1, -1, 2], [-1, 0, 1]] -> Option D

4. Quick Check:

   All unique triplets = [[-2, 0, 2], [-1, -1, 2], [-1, 0, 1]] [OK]

Hint: Sort array, then use two pointers to find triplets [OK]

Common Mistakes:

• Including triplets with wrong sums
• Missing duplicates in output
• Ignoring sorting step