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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 [];
}

// O(n) time · O(n) space

function lengthOfLongest(s) {
  const seen = new Map();
  let start = 0, best = 0;
  for (let i = 0; i < s.length; i++) {
    const ch = s[i];
    if (seen.has(ch) && seen.get(ch) >= start) {
      start = seen.get(ch) + 1;
    }
    seen.set(ch, i);
    best = Math.max(best, i - start + 1);
  }
  return best;
}