こんなんでいいのかどうかわからんけど
void main ()
{
srand( (unsigned int)time(NULL) );
int i;
const int length = 20;
int index = 0;
int Seq1[length];
int Seq2[length];
for( i = 0; i < length; i++ ){
Seq1[i] = i;
Seq2[i] = 0;
}

// 抜けた数字
int out = rand() % length;
for( i = 0; i < length; i++ ){
if( out != i ){
Seq2[index] = Seq1[i];
index++;
}
}
printf( "抜けた数字:%d\n", out );
for(i = 0; i < length; i++)
printf("%d ", Seq2[i]);
printf("\n");
}