Sorting an array in C?

In C, you can use the built in qsort command:

int compare( const void* a, const void* b)
{
     int int_a = * ( (int*) a );
     int int_b = * ( (int*) b );

     if ( int_a == int_b ) return 0;
     else if ( int_a < int_b ) return -1;
     else return 1;
}

qsort( a, 6, sizeof(int), compare )

see: http://www.cplusplus.com/reference/clibrary/cstdlib/qsort/

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To answer the second part of your question: an optimal (comparison based) sorting algorithm is one that runs with O(n log(n)) comparisons. There are several that have this property (including quick sort, merge sort, heap sort, etc.), but which one to use depends on your use case.

As a side note, you can sometime do better than O(n log(n)) if you know something about your data – see the wikipedia article on Radix Sort