阿里3.23笔试

Posted 2021-03-04 xlsryj

只第一题过了80%，3.24复盘

第一题：
n个人，选任意多人组成一个队(至少一个)，再从中选一个队长，问多少种方案，模1e9；
过80%
思路：dp打表，找规律，n*(2^(n-1))

    private static int quickmi(long a, long b, long m) {
        long ans = 1;
        long base = a;
        while (b > 0) {
            if(b % 2 == 1) ans = ans * base % m;
            base = base * base % m;
            b >>= 1;
        }
        return (int) ans;
    }
    //打表
    private static void solve1(int n) {
        int[][] dp = new int[n+1][n+1];
        dp[0][0] = 1;
        int res = 0;
        for(int i = 1; i <= n; i++) {
            for(int j = 1; j <= i; j++) {
                dp[i][j] = i - 1 >= j ? dp[i-1][j] : 0;
                dp[i][j] += dp[i-1][j-1];
                dp[i][j] += j > 1 ? dp[i-1][j-1] / (j - 1) : 0;
                res += dp[i][j];
            }
            System.out.println(res/n);
        }
    }
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long n = scanner.nextLong();
        long mod = 1000000007;
        long tmp = quickmi(2, n - 1, mod);
        System.out.println(((n % mod) * (tmp % mod)) % mod);
        

第二题：
走迷宫，不能走到障碍物上，每次上下左右移动一格，或者用飞行器飞到中心对称点(最多用五次)，最少多少次到达。

    static int[][] dir = new int[][]{{0, 1}, {1, 0}, {-1, 0}, {0, -1}};
    private static int bfs(char[][] matrix, int n, int m, int sx, int sy, int ex, int ey) {
        boolean[][][] visited = new boolean[n][m][6];
        Queue<int[]> que = new LinkedList<>();
        que.offer(new int[]{sx, sy, 0});
        int res = 0;
        while(!que.isEmpty()) {
            for(int size = que.size(); size > 0; size--) {
                int[] pos = que.poll();
                if(pos[0] == ex && pos[1] == ey) return res;
                visited[pos[0]][pos[1]][pos[2]] = true;
                for(int[] d : dir) {
                    int x = pos[0] + d[0], y = pos[1] + d[1];
                    if(x >= 0 && x < n && y >= 0 && y < m && matrix[x][y] != ‘#‘ && !visited[x][y][pos[2]]) {
                        que.offer(new int[]{x, y, pos[2]});
                    }
                }
                if(pos[2] < 5) {
                    int dx = n - 1 - pos[0], dy = m - 1 - pos[1];
                    que.offer(new int[]{dx, dy, pos[2] + 1});
                }
            }
            res++;
        }
        return -1;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int m = scanner.nextInt();
        scanner.nextLine();
        char[][] matrix = new char[n][m];
        for(int i = 0; i < n; i++) matrix[i] = scanner.nextLine().toCharArray();

        int sx = 0, sy = 0, ex = 0, ey = 0;
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                if(matrix[i][j] == ‘S‘) {
                    sx = i;
                    sy = j;
                }
                if(matrix[i][j] == ‘E‘) {
                    ex = i;
                    ey = j;
                }
            }
        }
        System.out.println(bfs(matrix, n, m, sx, sy, ex, ey));
    }