归并排序

function mergeSort(arr) {
  var len = arr.length;
  // 拆分
  if (len < 2) {
    return arr;
  }
  let middle = Math.floor(len / 2);
  const left = arr.slice(0, middle);
  const right = arr.slice(middle);

  return merge(mergeSort(left), mergeSort(right));
}

function merge(left, right) {
  //组合
  let result = [];
  while (left.length && right.length) {
    if (left[0] >= right[0]) {
      result.push(right.shift());
    } else {
      result.push(left.shift());
    }
  }

  while (left.length) {
    result.push(left.shift());
  }

  while (right.length) {
    result.push(right.shift());
  }
  return result;
}