Two Sum 15 XP Easy

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This is THE most famous coding interview question — LeetCode #1: Two Sum. If you're going to master one problem, make it this one.

Given an array of integers nums and an integer target, return the indices of the two numbers that add up to the target.

Example:
nums = [2, 7, 11, 15], target = 9
→ [0, 1] because nums[0] + nums[1] = 2 + 7 = 9

Brute Force — O(n²): Try every pair.

# Brute force — slow!
def twoSum(nums, target):
    for i in range(len(nums)):
        for j in range(i+1, len(nums)):
            if nums[i] + nums[j] == target:
                return [i, j]

Optimal — O(n) using HashMap:
The key insight: if we need a + b = target, then b = target - a. We can look up b in a hash map in O(1)!

💭 Challenge time! Now implement this yourself using the strategy above. The best way to learn is by doing! ⭐

How it works step by step:

i=0, num=2: complement=7, not in map → store {2: 0}
i=1, num=7: complement=2, 2 IS in map at index 0 → return [0, 1] ✓

Complexity: Time O(n), Space O(n)

📋 Instructions

Implement `twoSum(nums, target)` using the **hash map approach**. For each number, calculate the complement (target - num). Check if the complement exists in your hash map. If yes, return both indices. If no, store the current number and its index. Return the indices as a list `[i, j]` where `i < j`.

Use a dictionary: hashmap = {}. For each number, calculate complement = target - num. If complement is in hashmap → return both indices. Otherwise, store num: index.

⚠️ Try solving it yourself first — you'll learn more!

def twoSum(nums, target):
    hashmap = {}  # value → index
    for i, num in enumerate(nums):
        complement = target - num
        if complement in hashmap:
            return [hashmap[complement], i]
        hashmap[num] = i

🧪 Test Cases

Input

twoSum([2, 7, 11, 15], 9)

Expected

[0, 1]

Test case 1

Input

twoSum([3, 2, 4], 6)

Expected

[1, 2]

Test case 2

Input

twoSum([3, 3], 6)

Expected

[0, 1]

Test case 3

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main.py

Hi! I'm Rex 👋

Output

Ready. Press ▶ Run or Ctrl+Enter.

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