c++ - I write a the merge sort, but it has some bug -

i write small project implement merge sort. code:

template <typename t> void merge(t arr[], int begin, int mid, int end) {     int len = end - begin + 1;     t *temp = new t[len];     int = begin;     int j = mid + 1;     int k = 0;     while (i <= mid && j <= end)     {         if(arr[i] <= arr[j])             temp[k++] = arr[i++];         else             temp[k++] = arr[j++];     }     while (i <= mid)         temp[k++] = arr[i++];     while(j <= end)         temp[k++] = arr[j++];      memcpy(arr + begin, temp, len*sizeof(t)); }   //merge sort template <typename t> void mergesort(t arr[], int begin, int end) {     if (begin >= end)         return;      int mid = (end + begin) / 2;     mergesort(arr, begin, mid);     mergesort(arr, mid + 1, end);     merge(arr, begin, mid, end); }  int main() {     const int n = 10;     int arr[n];     for_each(arr, arr + n, [](int &val){ val = rand() % 100; });      copy(arr, arr+n, ostream_iterator<int>(cout, " "));     cout<<endl;      mergesort(arr, 0, n - 1);      copy(arr, arr+n, ostream_iterator<int>(cout, " "));     cout<<endl; } 

sometime got right answer, sometime got wrong answer. problem confuses me lot of time, when can find bug?

yes , rand() implementation different in both platforms, old post in stackoverflow,pls refer link here