17

#include <stdio.h>

int main() {
    int n = 5;

    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            if (i == 1 || i == n || j == 1 || j == n)
                printf("*");
            else
                printf(" ");
        }
        printf("\n");
    }

    return 0;
}

19

#include <stdio.h>

int main() {
    int n, i, j;
    printf("Enter Number of Rows: ");
    scanf("%d", &n);
    for (i = 1; i <= n; i++) {
        for (j = i; j < n; j++)
            printf(" ");
        for (j = 1; j <= 2 * i - 1; j++)
            printf("*");
        printf("\n");
    }
    for (i = n - 1; i >= 1; i--) {
        for (j = i; j < n; j++)
            printf(" ");
        for (j = 1; j <= 2 * i - 1; j++)
            printf("*");
        printf("\n");
    }
    return 0;
}

20

#include <stdio.h>
int main() {
    int n, i, j;
    printf("Enter n: ");
    scanf("%d", &n);
    for (i = 1; i <= n; i++) 
    {
        for (j = 1; j <= i; j++)
            printf("*");
        for (j = 1; j <= 2 * (n - i); j++)
            printf(" ");
        for (j = 1; j <= i; j++)
            printf("*");
        printf("\n");
    }
    for (i = n; i >= 1; i--) 
    {
        for (j = 1; j <= i; j++)
            printf("*");
        for (j = 1; j <= 2 * (n - i); j++)
            printf(" ");
        for (j = 1; j <= i; j++)
            printf("*");
        printf("\n");
    }
    return 0;
}