左程云大厂算法刷题班——15

第一题

leetcode499：迷宫2
思路：宽度优先遍历

public class 第一题15 {
    public static void main(String[] args) {
        int[][] maze = {{0,0,0,0,0},{1,1,0,0,1},{0,0,0,0,0},{0,1,0,0,1},{0,1,0,0,0}};
        int[] hole = {0,1};
        int[] ball = {4,3};
        String res = findShortestWay(maze, hole, ball);
        System.out.println(res);
    }
    public static int[][] to = {{1,0},{0,-1},{0,1},{-1,0},{0,0}};
    public static String[] re = {"d","l","r","u"};
    static class Node{
        public int x;
        public int y;
        public int d;//0,1,2,3,4   4 代表初始位置
        public String p;//路径
        public Node(int x,int y,int d,String p){
            this.x = x;
            this.y = y;
            this.d = d;
            this.p = p;
        }
    }
    public static String findShortestWay(int[][] maze,int[] hole,int[] ball){
        int n = maze.length;
        int m = maze[0].length;
        boolean[][][] visit = new boolean[n][m][4];
        int s1 = 0;
        int s2 = 0;
        Node[] p1 = new Node[n * m];
        Node[] p2 = new Node[n * m];
        s1 = spread(maze,n,m,visit,new Node(ball[0],ball[1],4,""),p1,s1);
        while (s1 != 0){
            for (int i = 0;i < s1;i++){
                Node cur = p1[i];
                if (cur.x == hole[0] && cur.y == hole[1]){
                    return cur.p;
                }
                s2 = spread(maze,n,m,visit,cur,p2,s2);
            }
            Node[] temp = p1;
            p1 = p2;
            p2 = temp;
            s1 = s2;
            s2 = 0;
        }
        return "impossible";
    }

    private static int spread(int[][] maze, int n, int m, boolean[][][] visit, Node cur, Node[] p2, int s2) {
        int d = cur.d;
        int x = cur.x + to[d][0];
        int y = cur.y + to[d][1];
        if (d == 4 || x < 0 || x == n || y < 0 || y == m || maze[x][y] != 0){
            for (int i = 0;i < 4;i++){
                if (i != d){
                    x = cur.x + to[i][0];
                    y = cur.y + to[i][1];
                    if (x >= 0 && x < n && y >= 0 && y < m && !visit[x][y][i]){
                        visit[x][y][i] = true;
                        p2[s2++] = new Node(x,y,i,cur.p + re[i]);
                    }
                }
            }
        }else {
            if (!visit[x][y][d]){
                visit[x][y][d] = true;
                p2[s2++] = new Node(x,y,d,cur.p);
            }
        }
        return s2;
    }
}

第二题

leetcode446. 等差数列划分 II - 子序列

public class 第二题15 {
    public static int numberOfArithmeticSlices(int[] nums) {
        int n = nums.length;
        if (n < 3){
            return 0;
        }
        HashMap<Long,Integer>[] dp = new HashMap[n];
        for (int i = 0;i < n;i++){
            dp[i] = new HashMap<>();
        }
        int res = 0;
        for (int i = 1;i < n;i++){
            for (int j = 0;j < i;j++){
                long diff = (long) nums[i] - nums[j];
                if (diff > Integer.MAX_VALUE || diff < Integer.MIN_VALUE){
                    continue;
                }
                dp[i].put(diff,dp[i].getOrDefault(diff,0) + dp[j].getOrDefault(diff,0) + 1);
                if (dp[j].containsKey(diff)){
                    res += dp[j].get(diff);
                }
            }
        }
        return res;
    }
}

第三题

leetcode489. 扫地机器人

public class 第三题15 {
    interface Robot{
        public boolean move();
        public void turnLeft();
        public void turnRight();
        public void clean();
    }
    public static int[][] ds = {{-1,0},{0,1},{1,0},{0,-1}};
    public static void cleanRoom(Robot robot){
        clean(robot,0,0,0,new HashSet<String>());
    }

    private static void clean(Robot robot, int x, int y, int d, HashSet<String> visited) {
        robot.clean();
        visited.add(x + "_" + y);
        for (int i = 0;i < 4;i++){
            int nd = (d + i) % 4;
            int nx = x + ds[i][0];
            int ny = y + ds[i][1];
            if (robot.move() && !visited.contains(nx + "_" + ny)){
                clean(robot,nx,ny,nd,visited);
            }
            robot.turnRight();
        }
        robot.turnRight();
        robot.turnRight();
        robot.move();
        robot.turnRight();
        robot.turnRight();
    }
}

第四题

leetcode875. 爱吃香蕉的珂珂
思路：二分

public class 第四题15 {
    public static void main(String[] args) {
        int i = 805306368 * 3;
        System.out.println(i);
    }
    public int minEatingSpeed(int[] piles, int h) {
        int maxValue = 1;
        for (int pile : piles){
            maxValue = Math.max(pile,maxValue);
        }
        int l = 1;
        int r = maxValue;
        int lastTime = maxValue;
        while (l <= r){
            int mid = (l + r) / 2;
            if (calculateSum(piles,mid) > h){
                l = mid + 1;
            }else {
                lastTime = mid;
                r = mid - 1;
            }
        }
        return lastTime;
    }

    private int calculateSum(int[] piles, int mid) {
        int res = 0;
        for (int i = 0;i < piles.length;i++){
            res += (piles[i] + mid - 1) / mid;
        }
        return res;
    }
}