Two Sum

Console

IntermediateArrays

Task

Implement `twoSum(nums, target)` returning the **indices** `[i, j]` (i < j) of the two numbers adding up to `target`. Return `null` when no pair exists. Exactly one pair exists in every passing test. The nested-loop version is O(n²); the classic upgrade uses a Map of seen values → index for O(n).

Hints

0 / 2

Brute force: for each i, scan j > i looking for nums[i] + nums[j] === target.
With a Map: while iterating, check if target - nums[i] was already seen and at which index.

Verification tests

twoSum([2,7,11,15], 9)→ [0,1]
twoSum([3,2,4], 6)→ [1,2]
twoSum([5,5], 10)→ [0,1]
twoSum([1,2,3], 99)→ null

⚠️ Reveal official solution

Try solving it yourself first — struggling is where the learning happens.

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 null;
}

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