Skip to main content

1. Two Sum

easyAsked at Lyft

Find two indices in an array whose values sum to a target.

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 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] <= 10^9
  • Only 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 of indices.

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 with its index; for every new value check if target-value exists. Single pass.

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:

Lyft-specific tips

Lyft uses this warm-up to confirm you reach for a hash map before brute force when discussing rider-driver lookup tables; verbalize the space-time tradeoff.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Two Sum and other Lyft interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →