How accelerate reduce and forEach method?

Question

I wrote a program to count sum of range of array items, but still can not pass the quiz because my code is slow: Execution Timed Out (12000 ms). How can I speed up my code to pass the quiz?

function maxSum(arr, range) {
  let result = -Infinity
  range.forEach(el => {
    let sumArr = arr.slice(el[0],el[1]+1).reduce((a,b) => a+b)
    sumArr > result && (result = sumArr)
  })
  console.log(result)
return result 
}
maxSum([1,-2,3,4,-5,-4,3,2,1],[[0,8],[1,3],[0,4],[6,8]])

Answer 1

Here's a faster and simpler solution. Iterates over range just once and collects all the sum totals, then maps it into sum ranges, then returns the max:

function maxSum(arr, range) {
  let left = [0];
  let total = 0;
  
  for (let num of arr) {
    left.push(total += num);
  }

  let sums = range.map(([a, b]) => left[b + 1] - left[a]);
  
  let result = Math.max(...sums);
  console.log(result);
  return result;
}

maxSum([1, -2, 3, 4, -5, -4, 3, 2, 1], [[0, 8], [1, 3], [0, 4], [6, 8]]);