Day72.java

import java.util.*;

public class Day72 {

    public List<List<String>> solveNQueens(int n) {
        List<List<String>> result = new ArrayList<>();
        char[][] board = new char[n][n];
        for (int i = 0; i < n; i++) {
            Arrays.fill(board[i], '.');
            
        }
        solveNQueensHelper(board, 0, result);
        return result;
    }

    private void solveNQueensHelper(char[][] board, int row, List<List<String>> result) {
        if (row == board.length) {
            result.add(constructSolution(board));
            return;
        }
        for (int col = 0; col < board.length; col++) {
            if (isValidPlacement(board, row, col)) {
                board[row][col] = 'Q';
                solveNQueensHelper(board, row + 1, result);
                board[row][col] = '.';
            }
        }
    }

    private boolean isValidPlacement(char[][] board, int row, int col) {
        for (int i = 0; i < row; i++) {
            if (board[i][col] == 'Q') return false;
        }
        // Check upper-left diagonal
        for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
            if (board[i][j] == 'Q') return false;
        }
        // Check upper-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;
    }

    private List<String> constructSolution(char[][] board) {
        List<String> solution = new ArrayList<>();
        for (char[] row : board) {
            solution.add(new Stri