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Nqueens.java
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Nqueens.java
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public class Nqueens {
static int count = 0;
public static void print_board(char[][] board){
for (int i=0; i<board.length; i++){
for (int j=0; j<board[0].length; j++){
System.out.print(board[i][j] + " ");
}
System.out.println();
}
}
public static boolean is_safe(char[][] board, int rows, int cols){
// vertical up
for (int i=rows-1; i>=0; i--){
if (board[i][cols]=='Q') return false;
}
// left diagonal up
for (int i=rows-1, j=cols-1; i>=0 && j>=0; i--, j--){
if (board[i][j]=='Q') return false;
}
// right diagonal up
for (int i=rows-1, j=cols+1; i>=0 && j<board.length; i--, j++){
if (board[i][j]=='Q') return false;
}
return true;
}
public static void N_Queens(char[][] board, int rows){
// base case
if (rows == board.length) {
System.out.println("_______BOARD_______");
print_board(board);
count++;
return;
}
// recursion call
for (int j=0; j<board.length; j++){
if (is_safe(board, rows, j)){
board[rows][j] = 'Q';
N_Queens(board, rows+1);
board[rows][j] = 'X'; // backtracking step
}
}
}
public static void main(String[] args) {
int n = 4;
char[][] board = new char[n][n];
for (int i=0; i<board.length; i++){
for (int j=0; j<board[0].length; j++){
board[i][j] = 'X';
}
}
N_Queens(board, 0);
System.out.println("\n" + count);
}
}