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1. Two Sum

easyAsked at LINE

Find two indices in an array whose values sum to a target — LINE warms up new grads with this before chat-server design.

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

Problem

Given an integer array nums and an integer target, return indices of the two numbers such that they add up to target. Assume exactly one solution exists and you cannot 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

Try every pair (i, j) and check if nums[i] + nums[j] === target.

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 single pass

Walk the array once, store value->index in a map, and for each element check if target - value exists.

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);
  }
  return [];
}

Tradeoff:

LINE-specific tips

At LINE, talk about how this hash-map lookup is the same primitive you'd use to dedupe message IDs in a chat history sync — concrete chat-app framing wins points.

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