Question

I recently took part in ACM certified programming competition. This is the question which I could not do at that time:

“Given an array of integers having n elements, write a program to print all the permutations.”

Please tell me how to do this question. Is there any algorithm to do this kind of questions?

Answer 1

assuming there are no repeats: just change each element with all possible following elements, and recursively invoke the function.

void permute(int *array,int i,int length) { 
  if (length == i){
     printArray(array,length);
     return;
  }
  int j = i;
  for (j = i; j < length; j++) { 
     swap(array+i,array+j);
     permute(array,i+1,length);
     swap(array+i,array+j);
  }
  return;
}

You can see the code with auxilary functions swap() and printArray() performing with a basic test case at ideone

Bonus: This is similar to the idea of fisher-yates shuffle, but in here – intead to swapping the element at i with randomly chosen following element – you swap it with all of them – each at a time.