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Problem solution · Java

Robot Room Cleaner

Robot Room Cleaner: a Java solution using depth-first search. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Depth-first search
Source
walkccc LeetCode Solutions
Length
50 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Depth-first search

For Robot Room Cleaner, the implementation follows one branch at a time, making it suitable for components, trees, backtracking, or dependency exploration.

  1. Define the state carried into one recursive or stack frame.
  2. Mark or choose the current state before exploring children.
  3. Combine child results or undo the choice when the branch finishes.

Code notes

  • 50 lines of Java from the credited upstream file 489.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • 1 loop block detected, together with recursive traversal.

Complexity

Count unique states for graph traversal; for backtracking, count the branching factor and maximum depth.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeRobot Room Cleaner · JavaJava
Use this to learn the idea, then write your own version.
/** * // 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) {    dfs(robot, 0, 0, 0, new HashSet<>());  }   private static final int[][] DIRS = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};   private void dfs(Robot robot, int d, int i, int j, Set<Pair<Integer, Integer>> seen) {    seen.add(new Pair<>(i, j));    robot.clean();     // Explore clockwise: 0: ^, 1: >, 2: v, 3: <    // The order is important since the idea is always turning right.    for (int k = 0; k < 4; ++k) {      final int newD = (d + k) % 4;      final int x = i + DIRS[newD][0];      final int y = j + DIRS[newD][1];      if (!seen.contains(new Pair<>(x, y)) && robot.move()) {        dfs(robot, newD, x, y, seen);        // Go back to the previous cell.        robot.turnRight();        robot.turnRight();        robot.move();        // Go back to the original direction.        robot.turnRight();        robot.turnRight();      }      robot.turnRight(); // Always turn the robot clockwise.    }  }}

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