(vc++6.0编译通过):
#include<stdio.h>
int main(void)
{
    int i, j, number;

    printf("Input your number: ");
    scanf("%i", &number);

    for(i = 0; i < (2*number-1); ++i)
    {

        if( i < number -1)
        {
            for(j = 0; j < (2*number-1); ++j)
            {
                if( j == number-1 - i  || j == number-1 + i )
                    printf("*");
                else
                    printf(" ");
            }
            printf("\n");
        }
        else if( i == number -1)
        {
            for( j = 0; j < (2*number-1); ++j)
            {
                if( j == number-1-i  || j == number-1 +i )
                    printf("*");
                else
                    printf(" ");
            }
            printf("\n");
        }
        else
        {
            for( j = 0; j < (2*number-1); ++j)
            {
                if( ((i - number) == (j-1)) || ((i - number) == ( 2*number-3 - j)) )
                    printf("*");
                else
                    printf(" ");
            }
            printf("\n");
        }

    }

    return 0;
}