/* ﾓｼﾞﾚﾂｦ ｹﾂｺﾞｳｽﾙ */
/* ｵｿｲ O( n^2 ) */
void conc1( int n, char** elem, char* result ) {
    int i;
    result[0] = '\0';
    for( i = 0; i < n; i++ )
        strcat( result, elem[i] );
}
/* ﾊﾔｲ O( n ) */
void conc2( int n, char** elem, char* result ) {
    int i;
    result[0] = '\0';
    char* p = result;
    for( i = 0; i < n; i++ )
        p += sprintf( p, "%s", elem[i] );
}