summaryRanges.js

summaryRanges.js

const summaryRanges = (nums, [beginning, end] = [nums[0]]) => {
  return nums.reduce((result, curr, i) => {
    const next = nums[i + 1];
    if (curr + 1 !== (next)) {
      result.push(end === beginning ? `${end}` : `${beginning}->${end}`);
      beginning = next;
    }
    end = next;
    return result;
  }, []);
}