程序代码：

#include <stdio.h>
#include <stdlib.h>

void prt_num(int *num, int n) {
    int i;

    for(i = 0; i < n; i++)
        if(num[i]) putchar(i + 'a');

    printf("  ");
}

int check(int *num, int n, int m) {
    int i, flag = 0;

    for(i = 0; i < n - m; i++)
        if(num[i]) flag = 1;

    return flag;
}

void comb_num(int *num, int n, int m) {
    int i, j, count;
    prt_num(num, n);

    for(;;) {
        count = 0;

        for(i = 0; i < n - 1; i++) {
            if(num[i] == 1 && num[i + 1] == 0) {
                num[i] = 0;
                num[i + 1] = 1;
                break;
            }

            if(num[i]) count++;
        }

        for(j = 0; j < i; j++) {
            if(j < count) num[j] = 1;

            else num[j] = 0;
        }

        prt_num(num, n);

        if(!check(num, n, m)) break;
    }
}

int main(void) {
    int n, m, *num, i;

    if((scanf("%d %d", &n, &m)) != 2 || m <= 0 || n <= m) return 1;

    if(!(num = malloc(sizeof(int) * n))) return 1;

    for(i = 0; i < n; i++) {
        if(i < m) num[i] = 1;

        else num[i] = 0;
    }

    comb_num(num, n, m);
    puts("");
    return 0;
}

程序代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void comb(char *s, int n, int m) {
    int i, j, k, t;
    int total = 1 << n;
    char **new = NULL;

    if(!(new = malloc(total * sizeof(char*)))) exit(EXIT_FAILURE);

    for(i = 0; i < total; i++)
        if(!(new[i] = calloc(n + 1, sizeof(char)))) exit(EXIT_FAILURE);

    if(n == 0) return;

    for(i = 1; i < total; i++) {
        for(k = 0, j = 0; j < n; j++) {
            t = i;

            if(t & (1 << j)) {
                new[i][k++] = *(s + j);
            }
        }
    }

    for(i = 1; i < total; i++)
        if(strlen(new[i]) == m)
            printf("%s\t", new[i]);

    for(i = 0; i < total; i++)
        free(new[i]);

    free(new);
}

int main(void) {
    char s[] = "12345";
    int n = strlen(s), m = 3;
    comb(s, n, m);
    return 0;
}

程序代码：

#include <stdio.h>
#include <string.h>

#define N 3
#define M 5

int combination[N+1] = {0};
void recursion(int m, int depth);

int main(void)
{
    recursion(1, 1);
    getchar();
    return 0;
} 

void recursion(int m, int depth)
{
    int i, j, k;

 
    for (i = m; i <= M; i++)
    {
        combination[depth] = i;
  
        if (depth == N)
        {
            for (j = 1; j <= N; j++)
            {
                printf("%d ", combination[j]);
            }
            printf("\n");
        }
        else
        {
            recursion(i+1, depth+1);
        }
    }
}