Leetcode51. N 皇后

题目传送地址: ​​https://leetcode.cn/problems/n-queens/​​

代码如下:

//递归解法
 public static List<List<String>> solveNQueens(int n) {
        List<List<String>> lists = fillChessboard(n, n);
        return lists;
    }

    //填充前row行
    public static List<List<String>> fillChessboard(int n, int row) {
        List<List<String>> result = new ArrayList<>();
        if (row == 1) {
            int i = 0;
            while (i < n) {
                StringBuilder stringBuilder = new StringBuilder();
                int j=0;
                while (j<n){
                    stringBuilder.append(".");
                    j++;
                }
                stringBuilder.replace(i, i + 1, "Q");
                result.add(Arrays.asList(stringBuilder.toString()));
                i++;
            }
            return result;
        }
        List<List<String>> list = fillChessboard(n, row - 1);
        for (List<String> list1 : list) {
            for (int col = 0; col < n; col++) {
                //校验该坐标处是否合适
                boolean validChessboard = validChessBoard(row, col, list1, n);
                if (validChessboard) {
                    StringBuilder stringBuilder = new StringBuilder();
                    int j=0;
                    while (j<n){
                        stringBuilder.append(".");
                        j++;
                    }
                    stringBuilder.replace(col, col + 1, "Q");
                    List<String> list2 = new ArrayList<>(list1);
                    list2.add(stringBuilder.toString());
                    result.add(list2);
                }
            }
        }
        return result;
    }

    //校验该坐标处是否合适
    public static boolean validChessBoard(int row, int col, List<String> chessboard, int n) {
        //校验行
        //校验列
        int j = 0;
        while (j < chessboard.size()) {
            String str = chessboard.get(j);
            char c = str.charAt(col);
            if (c == 'Q') {
                return false;
            }
            j++;
        }

        //校验正斜线
        int rowIndex = chessboard.size() - 1;
        int colIndex = col - 1;
        while (rowIndex >= 0 && colIndex >= 0) {
            String str = chessboard.get(rowIndex);
            char c = str.charAt(colIndex);
            if (c == '.') {
                rowIndex--;
                colIndex--;
            } else {
                return false;
            }
        }

        //校验反斜线
         rowIndex = chessboard.size() - 1;
         colIndex = col + 1;
        while (rowIndex >= 0 && colIndex < n) {
            String str = chessboard.get(rowIndex);
            char c = str.charAt(colIndex);
            if (c == '.') {
                rowIndex--;
                colIndex++;
            } else {
                return false;
            }
        }
        return true;
    }