Complete Graph-1

[PrakarshKamal]

No description provided.

[super30admin]

Find the town Judge (TownJudge.java)

I should point out that while using two arrays is correct and efficient, it's possible to use one array as in the reference solution. However, the student's approach is clear and easy to understand.

Final

VERDICT: PASS

---

Ball in the Maze (TheMaze.java)

Your solution is on the right track, but there is a critical issue in the while loop condition. The condition while (r >= 0 && c >= 0 && r < n && c < m && maze[r][c] == 0) is incorrect because it allows the ball to continue rolling only if the current cell is 0. However, the ball should roll until it hits a wall (value 1) or goes out of bounds. The condition should check that the next cell is within bounds and is not a wall. But note: in the reference solution, the while loop continues as long as the next cell is within bounds and is not a wall. Your condition checks the current cell instead of the next cell when moving. Actually, in your code, you are incrementing r and c first and then checking the conditions. This means you are checking the next cell after moving. However, the condition should be: while the next cell (r, c) is within bounds and is not a wall (maze[r][c] != 1), then continue. So your condition is correct in terms of checking the next cell, but note that the condition maze[r][c] == 0 is equivalent to maze[r][c] != 1 only if the maze has no other values. Since the problem states that the maze only has 0 and 1, it is acceptable. However, the reference solution uses maze[r][c] != 1 which is more explicit. But the real issue is that your while loop does not simulate the rolling correctly. Let me explain:

In your code, you start at the current position and then move one step in the direction. Then you check if that new position is valid and empty. Then you continue moving. This is correct. However, when the while loop exits, you have moved one step too far (into a wall or out of bounds). So you subtract one step to get back to the last valid position. This is correct.

But wait: the condition in the while loop is maze[r][c] == 0. This means that if the next cell is a wall (1), the while loop will break. So it should work. However, what if the next cell is out of bounds? The condition checks bounds first, so if r or c is out of bounds, the while loop breaks. So it seems correct.

But there is a subtle error: when the ball rolls, it should only stop when it hits a wall. However, your while loop condition also breaks if the next cell is not empty (i.e., if it is a wall). So it should be correct. Why then might it fail?

Consider: when the ball is at a cell that is adjacent to a wall. For example, if the ball is at (i,j) and there is a wall at (i+1,j). Then when moving down, you set r = i+1, c = j. Then you check: r>=0, c>=0, r<n, c<m -> true. Then check maze[r][c] == 0? It is 1, so the while loop breaks. Then you subtract dir[0] and dir[1] to get back to (i,j). So the ball doesn't move. This is correct because the ball cannot move down from (i,j) because it would hit the wall immediately.

So the while loop logic is actually correct. Then why do I think there is an error? Let me compare with the reference solution.

The reference solution does:

while(r >= 0 && c >= 0 && r < m && c < n && maze[r][c] != 1){
    r += dir[0];
    c += dir[1];
}

Then it subtracts one step. This is exactly the same as the student's code.

So the student's code should be correct. But wait: the student's code uses maze[r][c] == 0 which is equivalent to maze[r][c] != 1 only if the maze has no other values. Since the problem says the maze has only 0 and 1, it is correct.

Therefore, the student's solution is correct. However, there is one more thing: the reference solution marks the starting point as visited by setting it to -1 in the maze. The student uses a separate visited array. This is also correct and actually better because it doesn't modify the input.

So the student's solution is correct. But let me test with the examples.

Example 1:
maze = [[0,0,1,0,0],[0,0,0,0,0],[0,0,0,1,0],[1,1,0,1,1],[0,0,0,0,0]], start = [0,4], destination = [4,4]
The student's code should return true.

Example 2:
start = [0,4], destination = [3,2] -> false.

The code should work.

But wait: in the student's code, when the ball rolls, it might end up at the same stopping point multiple times? But the visited array prevents that.

So the solution is correct.

However, there is a potential issue: the while loop might not handle the case when the ball starts at a wall? But the problem states: "the ball and the destination exist in an empty space". So start is always an empty space.

Another issue: the student's code does not check if the start is the same as destination? But the problem says "they will not be in the same position initially". So no need.

But the code does check at the beginning of processing a node: if the popped node is the destination, return true. This is correct.

So the student's solution is correct.

But why did I initially think there was an error? Because I misread the while loop condition. The student's condition is correct.

One more thing: the order of directions in the student's code is {1,0}, {0,1}, {0,-1}, {-1,0}. This is different from the reference solution, but it doesn't matter.

VERDICT: PASS