Given a robot cleaner in a room modeled as a grid.

Each cell in the grid can be empty or blocked.

The robot cleaner with 4 given APIs can move forward, turn left or turn right. Each turn it made is 90 degrees.

When it tries to move into a blocked cell, its bumper sensor detects the obstacle and it stays on the current cell.

Design an algorithm to clean the entire room using only the 
 
4 given APIs shown below.

interface Robot {
  // returns true if next cell is open and robot moves into the cell.
  // returns false if next cell is obstacle and robot stays on the current cell.
  boolean move();

  // Robot will stay on the same cell after calling turnLeft/turnRight.
  // Each turn will be 90 degrees.
 

  void turnLeft();
  void turnRight();

  // Clean the current cell.
  void clean();
}
Example:

Input:
room = [
  [1,1,1,1,1,0,1,1],
  [1,1,1,1,1,0,1,1],
  [1,0,1,1,1,1,1,1],
  [0,0,0,1,0,0,0,0],
  [1,1,1,1,1,1,1,1]
],
row = 1,
col = 3

Explanation:
All grids in the room are marked by either 0 or 1.
0 means the cell is blocked, while
 
 1 means the cell is accessible.
The robot initially starts at the position of row=1, col=3.
From the top left corner, its position is one row below and three columns right.
Notes:

The input is only given to initialize the room and the robot's position internally. You must solve this problem "blindfolded". In other words, you must control the robot using only the mentioned 4 APIs, without knowing the room layout and the initial robot's position.
The robot's initial position will always be in an accessible cell.
The initial direction of the robot will be facing up.
All accessible cells are connected, which means the all cells marked as 1 will be accessible by the robot.
Assume all four edges of the grid are all surrounded by wall.

參考了一個很清晰的DFS的code：

// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

class Solution{
    public void cleanRoom(Robot robot) {
        HashSet<String> visited = new HashSet<>();
        helper(robot, 0, 0, visited);
    }
    
    public void helper(Robot robot, int x, int y,  HashSet<String> visited){
        //store visted or block coordinate as string in visited set
        String key = x + ":" + y;
        if(visited.contains(key)){
            return;
        }
        robot.clean;
        visited.add(key);
        
        if(moveUp(robot)){
            helper(robot, x-1, y, visited);
            moveDown(robot);
        }
        if(moveDown(robot)){
            helper(robot, x+1, y, visited);
            moveUp(robot);
        }
        if(moveLeft(robot)){
            helper(robot, x, y-1, visited);
            moveRight(robot);
        }
        if(moveRight(robot)){
            helper(robot, x, y+1, visited);
            moveLeft(robot);
        }
    }
    
    public boolean moveUp(Robot robot){
        return robot.move();
    }
    public boolean moveDown(Robot robot){
        robot.turnLeft();
        robot.turnLeft();
        boolean res = robot.move();
        robot.turnRight();
        robot.turnRight();
        return res;
        
    }
    public boolean moveLeft(Robot robot){
        robot.turnLeft();
        boolean res = robot.move();   
        robot.turnRight();
        return res;
    }
    public boolean moveRight(Robot robot){
        robot.turnRight();
        boolean res = robot.move();   
        robot.turnLeft();
        return res;
    }
    
}

class Robot {
    boolean move() {
        /*Default method by moving one step on the current direction
        * will return true if move successfully*/
        return true;
    }
    void turnLeft() {
        /*change direction to +90*/
    }

    void turnRight() {
        /*change direction to -90*/
    }
    void clean() {
        /*Do clean*/
    }
}