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

Three Sum Problem All Unique Triplets

📖 Scenario: You are working on a financial app that analyzes a list of daily profit and loss numbers. You want to find all unique sets of three days where the total profit/loss sums to zero. This helps identify balanced periods.

🎯 Goal: Build a program that finds all unique triplets in a list of integers that add up to zero.

📋 What You'll Learn

Create a list called nums with the exact integers: -1, 0, 1, 2, -1, -4

Create a variable called result to store the unique triplets

Use a sorted version of nums to help find triplets

Implement the two-pointer technique inside a loop to find all triplets that sum to zero

Print the result list containing all unique triplets

💡 Why This Matters

🌍 Real World

Finding balanced sets of financial transactions or data points that sum to zero can help detect patterns or anomalies.

💼 Career

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

1

Create the list of numbers

Create a list called nums with these exact integers: -1, 0, 1, 2, -1, -4

DSA Python

# Create the list nums with the given integers
# Your code here

Hint

Use square brackets to create the list and separate numbers with commas.

2

Initialize the result list and sort the numbers

Create a variable called result as an empty list to store triplets. Then create a variable called nums_sorted that holds the sorted version of nums.

DSA Python

nums = [-1, 0, 1, 2, -1, -4]
# Initialize result as an empty list
# Create nums_sorted as sorted(nums)
# Your code here

Hint

Use [] for an empty list and the sorted() function to sort.

3

Find all unique triplets that sum to zero

Use a for loop with variable i to iterate over nums_sorted up to length minus 2. Inside the loop, skip duplicates by checking if i > 0 and nums_sorted[i] == nums_sorted[i-1]. Then set two pointers: left = i + 1 and right = len(nums_sorted) - 1. Use a while loop to move left and right pointers to find triplets where the sum of nums_sorted[i] + nums_sorted[left] + nums_sorted[right] is zero. Add unique triplets to result. Skip duplicates for left and right pointers as well.

DSA Python

nums = [-1, 0, 1, 2, -1, -4]
result = []
nums_sorted = sorted(nums)
# Use a for loop with i to find triplets
# Your code here

Hint

Use two pointers moving inward to find sums equal to zero. Skip duplicates to avoid repeated triplets.

4

Print the list of unique triplets

Print the variable result to display all unique triplets that sum to zero.

DSA Python

nums = [-1, 0, 1, 2, -1, -4]
result = []
nums_sorted = sorted(nums)

for i in range(len(nums_sorted) - 2):
    if i > 0 and nums_sorted[i] == nums_sorted[i - 1]:
        continue
    left = i + 1
    right = len(nums_sorted) - 1
    while left < right:
        total = nums_sorted[i] + nums_sorted[left] + nums_sorted[right]
        if total == 0:
            result.append([nums_sorted[i], nums_sorted[left], nums_sorted[right]])
            while left < right and nums_sorted[left] == nums_sorted[left + 1]:
                left += 1
            while left < right and nums_sorted[right] == nums_sorted[right - 1]:
                right -= 1
            left += 1
            right -= 1
        elif total < 0:
            left += 1
        else:
            right -= 1
# Print the result list
# Your code here

Hint

Use print(result) to show the final list of triplets.