function minPlatform(arr, dep) {
  let obj = new Object();
  let plat = 0;
  let result = 0;
  for (let i = 0; i < arr.length; i++) {
    obj[arr[i]] = "a";
    obj[dep[i]] = "d";
  }
  for (let i of Object.keys(obj)) {
    if (obj[i] == "a") {
      plat++;
    } else {
      plat--;
    }
    if (plat > result) {
      result = plat;
    }
  }
  return result;
}

minPlatform(
  [900, 940, 950, 1100, 1500, 1800],
  [910, 1200, 1120, 1130, 1900, 2000]
);