// This time N queens on a chess board, same basic backtracking, now show all solutions.

#include <iostream>

using namespace std;

void print(int board[], int size)
{ for (int r = 0; r < size; r += 1)
  { for (int c = 0; c < size; c += 1)
      if (board[r] == c)
        cout << "X";
      else
        cout << "-";
    cout << "\n"; }
  cout << "\n"; }

bool all_ok(int board[], int r)
{ for (int i = 0; i < r; i += 1)
    if (board[i] == board[r])
      return false;
    else if (i - board[i] == r - board[r])
      return false;
    else if (i + board[i] == r + board[r])
      return false;
  return true; }

void put_queen(int board[], int size, int r)
{ // the initial call is put_queen(b, 0);   
  // given that the queens on rows 0 to r - 1 have no conflicts, ...   
  // ... try to place a new queen in row r, ...   
  // ... then continue to fill the board in the same way

  if (r == size)
    print(board, size);
  else
    for (int c = 0; c < size; c += 1)
    { board[r] = c;
      if (all_ok(board, r))
        put_queen(board, size, r + 1); } }

int main(int argc, char * argv[])
{ int size = 8;
  if (argc > 1)
  { char * c;
    size = strtol(argv[1], & c, 10);
    if (* c != '\0' || size < 1)
    { cerr << argv[1] << " is not a usable number\n";
      exit(1); } }

  int * board = new int[size];
  put_queen(board, size, 0);
  delete [] board; }