[LeetCode] 51. N-Queens

ProblemThe n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.Given an integer n, return all distinct solutions to the n-queens puzzle.Each solution contains a distinct board configuration of the n-queens’ placement, where ‘Q’ and ‘.’ both indicate a queen and an empty space respectively.Example:Input: 4Output: [ [".Q..", // Solution 1 “…Q”, “Q…”, “..Q.”], ["..Q.", // Solution 2 “Q…”, “…Q”, “.Q..”]]Explanation: There exist two distinct solutions to the 4-queens puzzle as shown above.Solutionclass Solution { public List<List<String>> solveNQueens(int n) { char[][] board = new char[n][n]; for (int i = 0; i < n; i++) { Arrays.fill(board[i], ‘.’); } List<List<String>> res = new ArrayList<>(); dfs(board, 0, res); return res; } private void dfs(char[][] board, int col, List<List<String>> res) { if (col == board.length) { res.add(construct(board)); return; } for (int row = 0; row < board.length; row++) { if (validate(board, row, col)) { board[row][col] = ‘Q’; dfs(board, col+1, res); board[row][col] = ‘.’; } } } private boolean validate(char[][] board, int row, int col) { for (int i = 0; i < board.length; i++) { for (int j = 0; j < col; j++) { if (board[i][j] == ‘Q’ && ( i+j == row+col || row+j == col+i || row == i )) return false; } } return true; } private List<String> construct(char[][] board) { List<String> res = new ArrayList<>(); for (int i = 0; i < board.length; i++) { String str = new String(board[i]); res.add(str); } return res; }}