Codewars - Sum of Intervals - 4 kyu.js

// https://www.codewars.com/kata/52b7ed099cdc285c300001cd/train/javascript

function sumIntervals(intervals){
  intervals = intervals.sort(function(a, b) { // O(n)
    return a[0] - b[0];
  });
  intervals = intervals.reduce(function(acc, el, index, array) { // O(n)
    const anterior = array[index - 1];
    if (array.length > 1 && anterior !== undefined) {
      if (el[0] < acc[acc.length - 1]) {
        if (el[1] >= acc[acc.length - 1]) {
          acc[acc.length - 1] = el[1];
        }
      } else {
        acc.push(...el);
      }
    } else {
      acc.push(...el);
    }
    return acc;
  }, []);
  let result = 0;
  for (let i = 0; i < intervals.length - 1 ; i+=2) { // O(2n)
    result+=(intervals[i + 1] - intervals[i]);
  }
  return result;
} // O(n) + O(n) + O(2n) = 3O(n) = O(n)