c语言 8-7

1、

#include <stdio.h>

int fac(int x)
{
    if(x > 0)
        return x * fac(x - 1);
    else
        return 1;
}

int main(void)
{
    int n, r;
    puts("please input two integers.");
    do
    {
        printf("n = "); scanf("%d", &n);
        if(n < 1)
        {
            puts("the range of n >= 1.");
            continue;
        }
        printf("r = "); scanf("%d", &r);
        if(r < 1)
        {
            puts("the range of r >= 1.");
        }
        if(n < r)
        {
            puts("n  >= r."); 
        }
    }
    while( n < r || n < 1 || r < 1);
    
    printf("result: %d
", fac(n) / (fac(r) * fac(n - r)));
    return 0; 
}

2、

#include <stdio.h>

int fac(int x)
{
    if(x > 0)
        return x * fac(x - 1);
    else
        return 1;
}

int com(int x, int y)
{
    return fac(x) / (fac(y) * fac(x - y));
}

int main(void)
{
    int n, r;
    puts("please input two integers.");
    do
    {
        printf("n = "); scanf("%d", &n);
        if(n < 1)
        {
            puts("n >= 1");
            continue;
        }
        printf("r = "); scanf("%d", &r);
        if(r < 1)
        {
            puts("r >= 1");
        }
        if(n < r)
        puts("n >= r");
    } 
    while(n < r || n < 1 | r < 1);
    
    printf("result: %d
", com(n, r));
    return 0;
}

3、

#include <stdio.h>

int com(int x, int y)
{
    int tmp;
    if(x > 1 && y > 1)
    {
        tmp = com(x - 1, y - 1) + com(x - 1, y);
    }
    if(y == 0 || x == y)
    {
        tmp = 1;
    }
    if(y == 1)
    {
        tmp = x;
    }
    return tmp;
}

int main(void)
{
    int a, b;
    puts("please input two integers.");
    do
    {
        printf("a = "); scanf("%d", &a);
        if(a < 1)
        {
            puts("a >= 1");
            continue;    
        }
        printf("b = "); scanf("%d", &b);
        if(b < 1)
        {
            puts("b >= 1");
        }
        if(a < b)
            puts("a >= b");
    }
    while(a < b || a < 1 | b < 1);
    
    printf("result: %d
", com(a, b));
    return 0;
}