js归并排序实现

 let arr = [3,4,2,8,1,1,2,2,3,3,4,5];

      const merge = function(L,M,R) {
          let arr2=[];
          let i=0;
          let p1 = L;
          let p2 = M+1;
          while(p1<=M&&p2<=R) {
              arr2[i++] = arr[p1]>=arr[p2]?arr[p1++]:arr[p2++];
          }
          while(p1<=M) {
              arr2[i++] = arr[p1++];
          }
          while(p2<=R) {
              arr2[i++] = arr[p2++];
          }
          for(i =0;i<arr2.length;i++) {
              arr[L+i] = arr2[i];
          }
      }

      const mergeSort = function(L,R) {
          if(L === R) {
              return;
          }
          let mid = L + ((R-L)>>1);
          mergeSort(L,mid);
          mergeSort(mid+1,R)
          return merge(L,mid,R);
      }