I do not understand the following explanation for: h1 = [1,2,1,1] h2 = [1,1,2] h3 = [1,1] There are 4, 3, and 2 cylinders in the three stacks, with their heights in the three arrays. Remove the top 2 cylinders from h1 (heights = [1, 2]) and from h2 (heights = [1, 1]) so that the three stacks all are 2 units tall. Return 2 as the answer.

h1 (heights = [1, 2]) has height 3, while h2 (heights = [1, 1]) has height 2, so the answer should be 1

JavaScript Soution:

function equalStacks(h1, h2, h3) {
  let h1Total = h1.reduce((a, c) => a + c);
  let h2Total = h2.reduce((a, c) => a + c);
  let h3Total = h3.reduce((a, c) => a + c);

  function checkEquality(one, two, three) {
    if (one === two && two === three) {
      return true;
    }
    return false;
  }

  while (!checkEquality(h1Total, h2Total, h3Total)) {
    let numberToSubtract = 0;
    if (h1Total >= h2Total && h1Total >= h3Total) {
      numberToSubtract = h1.shift();
      h1Total -= numberToSubtract;
    } else if (h2Total >= h1Total && h2Total >= h3Total) {
      numberToSubtract = h2.shift();
      h2Total -= numberToSubtract;
    } else if (h3Total >= h1Total && h3Total >= h1Total) {
      numberToSubtract = h3.shift();
      h3Total -= numberToSubtract;
    }
  }

  return h1Total;
}

It's readable

def equalStacks(h1, h2, h3): h1.reverse() h2.reverse() h3.reverse()

stacks = [h1, h2, h3]

heights = [sum(stack) for stack in stacks]
while True:
    if len(set(heights)) == 1:
        return heights[0]
    else:
        idx = heights.index(max(heights))
        heights[idx] -= stacks[idx].pop()   

def equalStacks(h1, h2, h3):

sums = [sum(h1), sum(h2), sum(h3)]
stacks = [h1, h2, h3]

t = min(sums)

while sums[0] != sums[1] or sums[1] != sums[2] or sums[0] != sums[2]: 

    for i in range(len(sums)):
        while sums[i] > t:
            sums[i] -= stacks[i].pop(0)   

    t = min(sums)

return t