Two Sum Problem Classic Hash Solution in DSA Python - Build from Scratch

Two Sum Problem Classic Hash Solution

📖 Scenario: You are helping a friend who runs a small store. They want to find two items that together cost exactly a certain amount of money. You will write a program to find the positions of these two items in the price list.

🎯 Goal: Build a program that finds two numbers in a list that add up to a target value using a hash map (dictionary) for fast lookup.

📋 What You'll Learn

Create a list called prices with the exact values: [2, 7, 11, 15]

Create a variable called target and set it to 9

Use a dictionary called seen to store numbers and their indices while looping through prices

Find two indices i and j such that prices[i] + prices[j] == target

Print the list of the two indices as the final output

💡 Why This Matters

🌍 Real World

Finding two items in a list that add up to a specific value is useful in shopping apps, budgeting tools, and financial software.

💼 Career

This problem is a common coding interview question and helps develop skills in using hash maps for efficient searching.

1

Create the list of prices

Create a list called prices with these exact values: [2, 7, 11, 15]

DSA Python

# Create the list called prices with values [2, 7, 11, 15]
# Your code here

Hint

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

2

Set the target sum

Create a variable called target and set it to 9

DSA Python

prices = [2, 7, 11, 15]
# Create the variable target and set it to 9
# Your code here

Hint

Use the equals sign to assign the value 9 to the variable target.

3

Find two indices using a dictionary

Create an empty dictionary called seen. Use a for loop with variables i and num to iterate over enumerate(prices). Inside the loop, check if target - num is in seen. If yes, create a list result with seen[target - num] and i, then break the loop. Otherwise, add num as a key and i as its value to seen.

DSA Python

prices = [2, 7, 11, 15]
target = 9
# Create dictionary seen and use a for loop to find two indices
# Your code here

Hint

Use a dictionary to remember numbers and their positions. Check if the complement exists before adding the current number.

4

Print the result

Print the variable result to show the two indices found.

DSA Python

prices = [2, 7, 11, 15]
target = 9
seen = {}
for i, num in enumerate(prices):
    if target - num in seen:
        result = [seen[target - num], i]
        break
    seen[num] = i
# Print the result list
# Your code here

Hint

Use the print function to display the list of indices.