489. Robot Room Cleaner

Description

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.

Problem URL

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Solution

给一个robot类和一个二维数组表示的房间，机器人可以前进，clean，左转和右转，将房间清理干净。

We use a backtrack recursive method to solve this problem. Maintaining a hash set to contain the position that we have visited. Clean it up and add it into set. Then for the four direcrtions, move forward and continue backtrack, after that, return to original position and maintain orginal direction. then turn to another direction.

Code

/**
 * // This is the robot's control interface.
 * // You should not implement it, or speculate about its implementation
 * interface Robot {
 *     // Returns true if the cell in front is open and robot moves into the cell.
 *     // Returns false if the cell in front is blocked and robot stays in the current cell.
 *     public boolean move();
 *
 *     // Robot will stay in the same cell after calling turnLeft/turnRight.
 *     // Each turn will be 90 degrees.
 *     public void turnLeft();
 *     public void turnRight();
 *
 *     // Clean the current cell.
 *     public void clean();
 * }
 */
class Solution {
    public void cleanRoom(Robot robot) {
        Set<String> set = new HashSet<>();
        backtrack(robot, set, 0, 0, 0);
    }
    
    private void backtrack(Robot robot, Set<String> set, int i, int j, int curDirection){
        String temp = i + "," + j;
        if (set.contains(temp)){
            return;
        }
        
        robot.clean();
        set.add(temp);
        
        for (int n = 0; n < 4; n++){
            if (robot.move()){
                int x = i, y = j;
                switch(curDirection){
                    case 0:{
                        x = i - 1;
                        break;
                    }
                    case 90:{
                        y = j + 1;
                        break;
                    }
                    case 180:{
                        x = i + 1;
                        break;
                    }
                    case 270:{
                        y = j - 1;
                        break;
                    }
                    default:{
                        break;
                    }
                }
                backtrack(robot, set, x, y, curDirection);
                //go back to orginal position
                robot.turnLeft();
                robot.turnLeft();
                robot.move();
                robot.turnRight();
                robot.turnRight();
            }
            //turn to next direction
            robot.turnRight();
            curDirection += 90;
            curDirection %= 360;
        }
    }
}

Time Complexity: O()
Space Complexity: O()

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