Для заданного n получить все возможные перестановки чисел 1,2,...n

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
 
unsigned find_i(const unsigned a[], unsigned n)
{
  unsigned i;
 
  assert(n > 0);
 
  for (i = n - 2; i != -1; --i)
    if (a[i] < a[i + 1])
      break;
 
  return i;
}
 
unsigned find_j(const unsigned a[], unsigned n, unsigned i)
{
  unsigned j;
 
  assert(n > 0 && i < n - 1);
 
  for (j = n - 1; j != i; --j)
    if (a[j] > a[i])
      break;
 
  return j;
}
 
void swap(unsigned a[], unsigned i, unsigned j)
{
  unsigned tmp = a[i];
  a[i] = a[j];
  a[j] = tmp;
}
 
void reverse(unsigned a[], unsigned n)
{
  assert(n > 0);
 
  for (unsigned i = 0, j = n - 1; i < j; ++i, --j)
    swap(a, i, j);
}
 
bool next_permutation(unsigned a[], unsigned n)
{
  assert(n > 0);
 
  unsigned i = find_i(a, n);
  if (i == -1)
    return false;
 
  assert(n > 1);
 
  unsigned j = find_j(a, n, i);
  swap(a, i, j);
  reverse(a + i + 1, n - i - 1);
 
  return true;
}
 
void print(unsigned a[], unsigned n)
{
  for (unsigned i = 0; i < n; ++i)
    printf("%u ", a[i]);
 
  printf("\n");
}
 
#define N 6u
 
int main()
{
  unsigned a[N];
 
  for (unsigned i = 0; i < N; ++i)
    a[i] = i + 1;
 
  do
    print(a, N);
  while (next_permutation(a, N));
}