程序代码：

#include <stdio.h>
#include <stdlib.h>
int a[27] = {0}, begin, end;
void dfs(int i, int len)
{
    if (len == 0)
    {
        for (i = begin; i <= end; ++i)
            if (a[i]) putchar('a' + i - 1);
        putchar('\n');
    }
    for (; i <= end; ++i)
    {
        if (a[i] == 0)
        {
            a[i] = 1;
            dfs(i + 1, len - 1);
            a[i] = 0;
        }
    }
}
int main(void)
{
    int len;
    while (scanf("%d%d%d", &begin, &end, &len) != EOF)
    {
        int nlen;
        dfs(begin, len > (nlen = end - begin + 1) ? nlen : len);
    }
    return 0;
}

程序代码：

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
void f(int *a,int end,int n,int x,int y);
int main(void)
{
    int start,end,num,n;
    int *a;
    scanf("%d %d %d",&start,&end,&num);
    n=(end-start+1)<num?(end-start+1):num;
    a=(int *)malloc(sizeof(int)*n);
    f(a,end,n,start,0);
    getch();
    return EXIT_SUCCESS;
}
void f(int *a,int end,int n,int x,int y)
{
    int i;
    a[y]=x;
    if(y==n-1)
    {
        for(i=0;i<n;i++)
            putchar(a[i]+'a'-1);
        printf("\n");
    }
    else
        if(y+1<n&&x+1<=end)
            f(a,end,n,x+1,y+1);
    if(x+1<=end)
        f(a,end,n,x+1,y);
    else
        return;
}