How to free 2d array in C?

You will have to loop over ptr[i], freeing each int* that you traverse, as you first suggest. For example:

for (int i = 0; i < N; i++)
{
    int* currentIntPtr = ptr[i];
    free(currentIntPtr);
}

Just the opposite of allocation:

for(int i = 0; i < N; i++)
    free(ptr[i]);
free(ptr);