Question

I saw this problem online, and I was trying to solve it in C++. I have the following algorithm:

char permutations( const char* word ){

  int size = strlen( word );
  if( size <= 1 ){
      return word;
  }
  else{
    string output = word[ 0 ];
    for( int i = 0; i < size; i++ ){
        output += permutations( word );
        cout << output << endl;
        output = word[ i ];
     }
  }
  return "";
}

For example, if I have abc as my input, I want to display abc, acb, bac, bca, cab, cba.
So, what I’m trying to do is

'abc' => 'a' + 'bc' => 'a' + 'b' + 'c'
                    => 'a' + 'c' + 'b'

so I need o pass a word less char every function call.
Could someone please help how to do it?

Answer 1

I suggest doing it using the algorithm header library in C++, much easier; and as a function can be written like this:

void anagram(string input){
    sort(input.begin(), input.end());
    do
        cout << input << endl;
    while(next_permutation(input.begin(), input.end()));
}

However since you want it without the STL, you can do it like so:

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

void swap (char *x, char *y)
{
    char ch = *x;
    *x = *y;
    *y = ch;
};

void permutate_(char* str, size_t index )
{
    size_t i = 0;
    size_t slen = strlen(str);
    char lastChar = 0;

    if (index == slen )
    {
        puts(str);
        return;
    }

    for (i = index; i < slen; i++ )
    {
        if (lastChar == str[i])
            continue;
        else
            lastChar = str[i];

        swap(str+index, str+i);
        permutate_(str, index + 1);
        swap(str+index, str+i);
    }
}

// pretty lame, but effective, comparitor for determining winner
static int cmpch(const void * a, const void * b)
{
    return ( *(char*)a - *(char*)b );
}

// loader for real permutor
void permutate(char* str)
{
    qsort(str, strlen(str), sizeof(str[0]), cmpch);
    permutate_(str, 0);
}

Which you can call by sending it a sorted array of characters,

permutate("Hello World");

The non-STL approach was gotten from here.