foo.cc: In function 'void MergeSort_(int, int)':
foo.cc:14:49: warning: operation on 'i' may be undefined [-Wsequence-point]
   14 |                 if(a[i+1]>=b[j+1]) num[i+j+l]=a[++i];
      |                                                 ^~~
foo.cc:14:49: warning: operation on 'i' may be undefined [-Wsequence-point]
foo.cc:15:35: warning: operation on 'j' may be undefined [-Wsequence-point]
   15 |                 else num[i+j+l]=b[++j];
      |                                   ^~~
foo.cc:15:35: warning: operation on 'j' may be undefined [-Wsequence-point]
foo.cc:17:35: warning: operation on 'i' may be undefined [-Wsequence-point]
   17 |         while(i!=la) num[i+j+l]=a[++i]; //b数组填完了 轮到a数组了
      |                                   ^~~
foo.cc:17:35: warning: operation on 'i' may be undefined [-Wsequence-point]
foo.cc:18:35: warning: operation on 'j' may be undefined [-Wsequence-point]
   18 |         while(j!=lb) num[i+j+l]=b[++j]; //a数组填完了 轮到b数组了
      |                                   ^~~
foo.cc:18:35: warning: operation on 'j' may be undefined [-Wsequence-point]

[Hydro](https://hydro.ac)提供评测服务