int num=1, largest = 0, second = 0;
bool isFirst = true;
printf( "請輸入數字，如要跳出，請輸入 0\n" );
while ( num != 0 )
{
　　scanf( "%d", &num );
　　if ( isFirst )
　　　　{ largest = num; second = num; isFirst = false; }
　　if ( num > largest )
　　　　{ second = largest; largest = num; }
　　else if ( num > second )
　　　　second = num;
}
printf( "輸入中最大和次大為 %d 和 %d ", largest, second );

int 型態本身就有含正數與負數了
不需要加 signed