Poison

52. N-Queens II

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class Solution {
    private static class CountHolder {
        private int count;
    }

    public int totalNQueens(int n) {
        CountHolder countHolder = new CountHolder();

        char[][] board = new char[n][n];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                board[i][j] = '.';
            }
        }

        dfs(countHolder, board, 0);
        return countHolder.count;
    }

    private void dfs(CountHolder countHolder, char[][] board, int rowIndex) {
        if (rowIndex == board.length) {
            countHolder.count++;
            return;
        }

        for (int j = 0; j < board.length; j++) {
            if (canFill(board, rowIndex, j)) {
                board[rowIndex][j] = 'Q';
                dfs(countHolder, board, rowIndex + 1);
                board[rowIndex][j] = '.';
            }
        }
    }

    private List<String> toList(char[][] board) {
        List<String> lineList = new ArrayList<>(board.length);
        for (char[] line : board) {
            lineList.add(new String(line));
        }
        return lineList;
    }

    private boolean canFill(char[][] used, int i, int j) {
        for (int k = 0; k <= i; k++) {
            if (used[k][j] == 'Q') {
                return false;
            }
        }

        for (int x = i - 1, y = j - 1; x >= 0 && y >= 0; x--, y--) {
            if (used[x][y] == 'Q') {
                return false;
            }
        }
        for (int x = i - 1, y = j + 1; x >= 0 && y < used.length; x--, y++) {
            if (used[x][y] == 'Q') {
                return false;
            }
        }

        return true;
    }
}
Reference

52. N-Queens II