Counting Sort (legacy C)

• https://en.wikipedia.org/wiki/Counting_sort

    #include <stdio.h>
    #include <strings.h>
    
    int main() {
        const char null = '\0';
        const char* alpha = "ACGT";
        const size_t N = 1e3,
                     M = strlen( alpha );
        char S[ N ] = { null }; scanf( "%s", S );
        size_t C[ 'T' + 1 ] = { 0 }; { // (C)ount of character instances in S
            for( int i=0; i < N && S[ i ] != null; ++i ){
                char ch = S[ i ];
                ++C[ ch ];
            }
        }
        char A[ N ] = { null }; { // (A)rray of characters from S in sorted order
            int i=0;
            for( int j=0; j < M; ++j ){
                char ch = alpha[ j ];
                while( C[ ch ]-- )
                    A[ i++ ] = ch;
            }
        }
        for( int i=0; i < N && A[ i ] != null; printf( "%c", A[ i++ ] ) ); printf( "\n" );
        return 0;
    }