function mergeSort(arr){
  return mergeSortRec(arr)
}
function mergeSortRec(arr){
  if(arr.length===1){
    return arr;
  }
  let mid = Math.floor(arr.length/2);
  let left = arr.slice(0,mid);
  let right = arr.slice(mid,arr.length)
  return merge(mergeSortRec(left), mergeSortRec(right))
}
function merge(left,right){
  let result = [],i=0,j=0;
  while(i<left.length && j<right.length){
    if(left[i]<right[j]){
      result.push(left[i++])
    }else{
      result.push(right[j++])
    }
  }
  while(i<left.length){
    result.push(left[i++])
  }
  while(j<right.length){
    result.push(right[j++])
  }
  return result;
}
// 测试用
let array = []
for(let i=0;i<10000;i++){
  array.push(Math.floor(Math.random()*10000))
}
console.time('快速排序耗时:');
console.log(mergeSort(array))
console.timeEnd('快速排序耗时:');