Coding Interview Preparation on Telegram: “🎯 CODING CHALLENGE #1

🎯 CODING CHALLENGE #1 - Two Sum
Difficulty: Easy | Asked at: Google, Amazon, Meta

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

python
nums = [2, 7, 11, 15]
target = 9
# Expected output: [0, 1]

You can't use the same element twice, and there's exactly one valid answer.

💡 Hint: Before you reach for a brute-force double loop, ask yourself - what if you could look up "have I seen the number I need?" in O(1)?

Try it yourself before scrolling for the solution 👇

Solution:

python
def two_sum(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        complement = target - num
        if complement in seen:
            return [seen[complement], i]
        seen[num] = i
    return []

Complexity: O(n) time, O(n) space - one pass, hash map lookup.

Common mistake: Candidates often solve this with nested loops (O(n²)) and stop there. If you already have the optimal solution, say it out loud early: "I can brute-force this in O(n²), but I think we can do better with a hash map." That sentence alone signals seniority.

What's the first approach that came to your mind? 🤔