day30 打卡332. 重新安排行程 51. N 皇后 37. 解数独

332. 重新安排行程

去b站搜了视频讲解在写的。视频地址

class Solution {
    List<String> result = new ArrayList<>();

    public List<String> findItinerary(List<List<String>> tickets) {
        //  from     to
        Map<String, TreeMap<String, Integer>> map = new HashMap<>();
        for (List<String> list: tickets) {
            String from = list.get(0);
            String to = list.get(1);

            map.putIfAbsent(from, new TreeMap<String, Integer>());
            TreeMap<String, Integer> treeMap = map.get(from);
            // 记住to的后面的机票往哪里去飞
            treeMap.put(to, treeMap.getOrDefault(to, 0) + 1);
        }
        // 初始化JFK
        result.add("JFK");
        backtracking(tickets, map, 0);
        return result;
    }

    private boolean backtracking(List<List<String>> tickets, Map<String, TreeMap<String, Integer>> map, int progress) {
        // 机票用完了
        if (progress == tickets.size()) {
            return true;
        }

        // 得到 每一次的开始点
        String from = result.get(result.size()-1);
        // 得到 从开始点可以到达的目标
        TreeMap<String, Integer> tos = map.get(from);

        if (tos == null || tos.isEmpty() || tos.size() == 0) {
            return false;
        }

        for (String key: tos.keySet()) {
            // 没票了
            if (tos.get(key) == 0) {
                continue;
            }

            result.add(key);
            tos.put(key, tos.get(key) - 1);

            if (backtracking(tickets, map, progress+1)) {
                return true;
            }

            result.remove(result.size()-1);
            tos.put(key, tos.get(key) + 1);
        }
        return false;
    }
}

51. N 皇后

51题目链接

class Solution {
    List<List<String>> result = new ArrayList<>();

    public List<List<String>> solveNQueens(int n) {
        if (n == 1) {
            result.add(Arrays.asList("Q"));
            return result;
        }
        char[][] chessboard = new char[n][n];
        for (char[] c: chessboard) {
            Arrays.fill(c, '.');
        }
        backtracking(chessboard, n, 0);
        return result;
    }

    private void backtracking(char[][] chessboard, int n, int row) {
        if (row == n) {
            // 收获结果，将chessboard转化成list
            result.add(getPath(chessboard));
            return;
        }

        for (int i = 0 ; i<n ; i++) {
            if (isValid(chessboard, n, row, i)) {
                // 符合要求
                chessboard[row][i] = 'Q';
                backtracking(chessboard, n, row+1);
                chessboard[row][i] = '.';
            } 
        }
    }

    private List<String> getPath(char[][] chessboard) {
        List<String> list = new ArrayList<>();
        for (char[] c: chessboard) {
            list.add(String.copyValueOf(c));
        }
        return list;
    }

    private boolean isValid(char[][] chessboard, int n, int row, int col) {
        // 检查列
        for (int i=0; i<row; ++i) { // 相当于剪枝
            if (chessboard[i][col] == 'Q') {
                return false;
            }
        }

        // 检查45度对角线
        for (int i=row-1, j=col-1; i>=0 && j>=0; i--, j--) {
            if (chessboard[i][j] == 'Q') {
                return false;
            }
        }

        // 检查135度对角线
        for (int i=row-1, j=col+1; i>=0 && j<=n-1; i--, j++) {
            if (chessboard[i][j] == 'Q') {
                return false;
            }
        }
        return true;
    }
}

37. 解数独

37题目链接

class Solution {
    public void solveSudoku(char[][] board) {
        backtracking(board);
    }

    private boolean backtracking(char[][] board) {
        int n = board.length;
        for (int i = 0 ; i<n ; i++) {
            for (int j = 0 ; j<n ; j++) {
                if (board[i][j] == '.') {
                    // 放数字
                    for (char k = '1'; k<='9' ; k++) {
                        if (isValid(board, i, j, k)) {
                            // 如果符合要求，进行回溯
                            board[i][j] = k;
                            boolean result = backtracking(board);
                            if (result) return true;
                            board[i][j] = '.';
                        }
                    }
                    // 如果9个数字都不行，返回false
                    return false;
                }
            }
        }
        return true;
    }

    private boolean isValid(char[][] board, int row, int col, char val) {
        // 判断一行是否有等于val
        for (int i = 0 ;i<board.length ; i++) {
            if (board[row][i] == val) return false;
        }

        // 判断一列是否有等于val
        for (int i = 0 ;i<board.length ; i++) {
            if (board[i][col] == val) return false;
        }

        // 判断所在九宫格是否有等于val
        int startX = (row / 3) * 3;
        int startY = (col / 3) * 3;
        for (int i = startX ; i < startX+3 ; i++) {
            for (int j = startY ; j < startY+3 ; j++) {
                if (board[i][j] == val) return false;
            }
        }
        return true;
    }
}