I started learning about backtracking in college and was asked to solve the famous 8 queens problem. I know there are many solutions out there for this, but I would like to get there by myself. However, I am kind of stuck and in need for some guidance here.

The problem is that I don't have quite understood how to backtrack properly.

When it comes to the point that there are no more legal positions at the board, I remove the last queen placed, but the algo will place the queen again at the same place that originated no valid positions. I thought about storing the sequences that are invalid so that they are not repeated but I am sure that's not the way to go about it.

Any opinion or pointer in the right direction is greatly appreciated.

Thanks.

```
package queens;
public class Queens
{
private int[][] board = new int[9][9];
private int numQueens = 0;
public void printBoard()
{
for (int i = 1; i <= 8; i++)
{
for (int j = 1; j <= 8; j++)
System.out.printf("%2d", board[i][j]);
System.out.print("\n");
}
System.out.print("\n");
}
public void place(int i, int j)
{
board[i][j] = 1;
numQueens++;
}
public boolean accepts(int i, int j)
{
int row, col;
row = 1; while (row <= 8) if (board[row++][j] == 1) return false;
col = 1; while (col <= 8) if (board[i][col++] == 1) return false;
row = i; col = j; while (row <= 8 && col <= 8) if (board[row++][col++] == 1) return false;
row = i; col = j; while (row <= 8 && col >= 1) if (board[row++][col--] == 1) return false;
row = i; col = j; while (row >= 1 && col <= 8) if (board[row--][col++] == 1) return false;
row = i; col = j; while (row >= 1 && col >= 1) if (board[row--][col--] == 1) return false;
return true;
}
public void remove(int i, int j)
{
board[i][j] = 0;
numQueens--;
}
public boolean isComplete()
{
return numQueens == 8;
}
public boolean calc()
{
if (isComplete())
return true;
int last_i = 0, last_j = 0;
for (int i = 1; i <= 8; i++)
{
for (int j = 1; j <= 8; j++)
{
if (accepts(i, j))
{
place(i, j);
printBoard();
last_i = i;
last_j = j;
}
}
}
remove(last_i, last_j);
calc();
}
}
```