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Copy pathNQueens.java
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66 lines (58 loc) · 1.78 KB
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// Time complexity is O(n!)
public class NQueens {
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.length; j++) {
board[i][j] = 'x';
}
}
nQueens(board, 0);
}
public static void nQueens(char[][] board, int row) {
// base condition
if (row == board.length) {
printBoard(board);
System.out.println("--------chess-board--------");
return;
}
// coloum loop
for (int col = 0; col < board.length; col++) {
if (isSafe(board, row, col)) {
board[row][col] = 'Q';
nQueens(board, row+1); // recursive condition
board[row][col] = 'x'; // backtracking step
}
}
}
public static boolean isSafe(char[][] board, int row, int col) {
// vertical
for (int i = row-1; i >= 0; i--) {
if (board[i][col] == 'Q') {
return false;
}
}
//up left diagonal
for (int i = row-1,j = col-1; i >= 0 && j >= 0 ; i--, j--) {
if (board[i][j] == 'Q') {
return false;
}
}
// up right diagonal
for (int i = row-1,j = col+1; i >= 0 && j < board.length ; i--, j++) {
if (board[i][j] == 'Q') {
return false;
}
}
return true;
}
public static void printBoard(char[][] board) {
for (int i = 0; i < board.length; i++) {
for (int j = 0; j < board.length; j++) {
System.out.print(board[i][j]+ " ");
}
System.out.println();
}
}
}