gistfile1.c

//
//  main.c
//  NumberPyramid
//
//  Created by jamie on 12/10/18.
//  Copyright (c) 2012年 jamie. All rights reserved.
//

#include <stdio.h>

static long epoch = 0;

void swap (long *x, long *y)
{
  long temp;
  temp = *x;
  *x = *y;
  *y = temp;
}

void print_bucket(long* bucket){
  int i=0;
  printf("%ld/%ld%ld+%ld/%ld%ld+%ld/%ld%ld\n", bucket[i++], bucket[i++], bucket[i++], bucket[i++], bucket[i++], bucket[i++], bucket[i++], bucket[i++], bucket[i++]);
}

void permute(long *a, int i, int n)
{
  int j;
  if (i == n){
    epoch++;
    long a1 = a[0];
    long a4 = a[3];
    long a7 = a[6];
    long a23 = 10*a[1]+a[2];
    long a56 = 10*a[4]+a[5];
    long a89 = 10*a[7]+a[8];
    if ( a1<a4 && a4<a7 && (a1*a56*a89 + a4*a23*a89 + a7*a23*a56)==a23*a56*a89 ){
      print_bucket(a);
    }
  }
  else
  {
    for (j = i; j <= n; j++)
    {
      swap((a+i), (a+j));
      permute(a, i+1, n);
      swap((a+i), (a+j)); //backtrack
    }
  }
}

int main(int argc, const char * argv[])
{
  long bucket[9]={1,2,3,4,5,6,7,8,9};
 
  permute(bucket, 0, 8);
  
  printf("After %ld permutations", epoch);
  
  return 0;
}