合算法：从m个数中选n个数的所有组合

#include <iostream>
using namespace std;
int combine(int a[], int n, int m)
{   
  m = m > n ? n : m;

  int* order = new int[m+1];    
  for(int i=0; i<=m; i++)
    order[i] = i-1;            

  int count = 0;                                
  int k = m;
  bool flag = true;           
  while(order[0] == -1)
  {
    if(flag)                  
    {   
      for(int i=1; i<=m; i++)
      {
        cout << a[order[i]] << " ";
      }
      cout << endl;
      count++;
      flag = false;
    }

    order[k]++;              
    if(order[k] == n)          
    {
      order[k--] = 0;
      continue;
    }     

    if(k < m)                  
    {
      order[++k] = order[k-1];
      continue;
    }

    if(k == m)
      flag = true;
  }

  delete[] order;
  return count;
}

int main()
{
  const int M = 3;
  const int N = 2;
  int a[M];
  int b[N];
  for(int i=0;i<M;i++)
    a[i] = i+1;

  cout<<combine(a,M,N)<<endl; 
  system("pause");
  return 0;
}