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Two-sum (hashmap), .

Finds two indices summing to a target using a lookup object.

two-sum.js
function twoSum(nums, target) {
  const seen = {};
  for (let i = 0; i < nums.length; i++) {
    const need = target - nums[i];
    if (seen[need] !== undefined) return [seen[need], i];
    seen[nums[i]] = i;
  }
  return [];
}

twoSum([2, 7, 11, 15], 26);

What to watch

The seen object grows key by key; watch it find the complement and return both indices.

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