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1. Two Sum

easyAsked at Instacart

Find two array indices whose values sum to a target — the warmup Instacart uses to gauge baseline hash-map fluency before pricing or inventory matching follow-ups.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each input has exactly one solution, and you may not use the same element twice.

Constraints

  • 2 <= nums.length <= 10^4
  • -10^9 <= nums[i], target <= 10^9
  • Exactly one valid answer exists

Examples

Example 1

Input
nums = [2,7,11,15], target = 9
Output
[0,1]

Example 2

Input
nums = [3,2,4], target = 6
Output
[1,2]

Approaches

1. Brute force

Check every pair until a match is found.

Time
O(n^2)
Space
O(1)
for (let i = 0; i < nums.length; i++) {
  for (let j = i+1; j < nums.length; j++) {
    if (nums[i] + nums[j] === target) return [i, j];
  }
}

Tradeoff:

2. Hash map

Store each value's index as you scan, looking up the complement in O(1).

Time
O(n)
Space
O(n)
function twoSum(nums, target) {
  const seen = new Map();
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen.has(need)) return [seen.get(need), i];
    seen.set(nums[i], i);
  }
}

Tradeoff:

Instacart-specific tips

Instacart wants you to articulate the time/space tradeoff out loud — they often follow up by asking how this pattern would apply to matching shopper IDs to open orders.

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Output

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