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NQueens.java
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NQueens.java
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package recursion;
public class NQueens {
public static void main(String[] args) {
int row = 4;
boolean[][] board = new boolean[row][row];
System.out.println(queens(board,0));
}
public static int queens(boolean[][] board, int row) {
if(row == board.length) {
display(board);
System.out.println();
return 1;
}
int count = 0;
//placing the queen and checking each row and column
for (int col = 0; col<board.length; col++) {
if(isSafe(board,row,col)) {
board[row][col] = true;
count += queens(board,row+1);
board[row][col] = false;
}
}
return count;
}
private static boolean isSafe(boolean[][] board, int row, int col) {
for(int i=0; i<row; i++) {
if(board[i][col])
return false;
}
//left diagonal
int i=row-1,j=col-1;
while (i>=0 && j>= 0) {
if(board[i][j])
return false;
i--;
j--;
}
//right diagonal
i=row-1;j=col+1;
while (i>=0 && j<board.length) {
if(board[i][j])
return false;
i--;
j++;
}
return true;
}
private static void display(boolean[][] board) {
for (boolean[] row: board) {
for (boolean element: row) {
if(element) {
System.out.print("Q ");
}else {
System.out.print("X ");
}
}
System.out.println();
}
}
}