Crawler Log Folder

Easy

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StacksStrings

The Leetcode file system keeps a log each time some user performs a change folder operation.

The operations are described below:

"../" : Move to the parent folder of the current folder. (If you are already in the main folder, remain in the same folder).
"./" : Remain in the same folder.
"x/" : Move to the child folder named x (This folder is guaranteed to always exist).

You are given a list of strings logs where logs[i] is the operation performed by the user at the i^th step.

The file system starts in the main folder, then the operations in logs are performed.

Return the minimum number of operations needed to go back to the main folder after the change folder operations.

Example 1:

Input: logs = ["d1/","d2/","../","d21/","./"]
Output: 2
Explanation: Use this change folder operation "../" 2 times and go back to the main folder.

Example 2:

Input: logs = ["d1/","d2/","./","d3/","../","d31/"]
Output: 3

Example 3:

Input: logs = ["d1/","../","../","../"]
Output: 0

Constraints:

1 <= logs.length <= 10³
2 <= logs[i].length <= 10
logs[i] contains lowercase English letters, digits, '.', and '/'.
logs[i] follows the format described in the statement.
Folder names consist of lowercase English letters and digits.

Try coding it in LeetCode

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

Can the folder names in the logs contain any special characters or spaces?
Is the root folder depth considered 0, and should I return 0 if I end up back at the root?
Are empty folder names (e.g., '//') valid log entries and how should they be handled?
Can I assume that the input `logs` list is never null or empty?
Are there any invalid sequences of operations I should be aware of, or can I assume the logs are always valid in terms of attempting to go above the root directory?

Brute Force Solution

Approach

The problem asks us to determine the final folder depth given a series of operations. A brute force approach simulates each operation one by one, updating the folder depth as we go to arrive at the final answer.

Here's how the algorithm would work step-by-step:

Start at the root folder, which has a depth of zero.
Go through the list of operations one at a time, in the order they are given.
For each operation, check what it says to do.
If the operation says to move into a subfolder, increase the folder depth by one.
If the operation says to move to the parent folder, decrease the folder depth by one, but only if the current depth is greater than zero (because you can't go below the root folder).
If the operation says to stay in the current folder (or do nothing else that affects the depth), do nothing to the folder depth.
After processing all the operations, the final folder depth is the answer.

Code Implementation

def crawler_log_folder_brute_force(logs):
    folder_depth = 0

    for log in logs:
        if log == "../":
            #  Move to the parent folder, if not at the root.
            if folder_depth > 0:
                folder_depth -= 1

        elif log == "./":
            # Stay in the current folder, so do nothing.
            pass

        else:
            # Move to a subfolder, increase folder depth.
            folder_depth += 1

    return folder_depth

Big(O) Analysis

Time Complexity

O(n) – The solution iterates through the input array of operations once. The number of operations to perform is directly proportional to the number of logs in the input. Therefore, the time complexity is O(n), where n is the number of operations in the input array.

Space Complexity

O(1) – The provided algorithm only uses a single integer variable to track the folder depth. Regardless of the number of operations (N), the space required for this variable remains constant. No additional data structures like arrays, lists, or hash maps are used. Therefore, the auxiliary space complexity is O(1).

Optimal Solution

Approach

We need to figure out the final folder depth given a series of operations. We can simulate the operations while keeping track of the current folder level. The key is to efficiently handle the "move to parent folder" operation.

Here's how the algorithm would work step-by-step:

Start at the main folder (folder level 0).
Go through the list of actions one by one.
If the action is "stay in the current folder", do nothing.
If the action is "move to the parent folder", go back one level, but don't go below the main folder level.
If the action is "move to a subfolder", go forward one level.
After going through all the actions, report the final folder level.

Code Implementation

def crawler_log_folder(logs):
    folder_depth = 0

    for log in logs:
        if log == "../":
            # Move to the parent folder, if possible.
            folder_depth = max(0, folder_depth - 1)

        elif log == "./":
            # Stay in the current folder; do nothing.
            pass

        else:
            # Move to a subfolder.
            folder_depth += 1

    return folder_depth

Big(O) Analysis

Time Complexity

O(n) – The algorithm iterates through the input array of logs once. For each log entry, a constant amount of work is performed: either incrementing a counter, decrementing a counter (with a check to prevent going below zero), or doing nothing. Therefore, the time complexity is directly proportional to the number of logs, n.

Space Complexity

O(1) – The algorithm uses a single integer variable to keep track of the folder depth. The space required for this variable remains constant regardless of the number of operations, denoted as N. No auxiliary data structures like arrays, lists, or hashmaps are used. Therefore, the auxiliary space complexity is constant, or O(1).

Edge Cases

Case	How to Handle
Empty input log array	Return 0, as no moves are made, implying we are already at the main folder.
Logs contain only './' operations	Return 0, as './' operations do not change the current folder, so we remain at the main folder.
Logs contain only '../' operations, resulting in going beyond the main folder	Handle '../' operations by decrementing the depth only if depth > 0; otherwise, stay at depth 0.
Logs contain a mix of '../' and child folder operations leading back to the main folder	The algorithm accurately tracks the folder depth, returning to 0 after navigating back up.
Logs contain extremely deep child folder nesting	The integer representing depth should handle a very large positive value representing the nesting level.
Logs contain invalid operation strings (e.g., 'abc/')	Treat any string that doesn't match '../', './', or 'x/' as a child folder move ('x/'), effectively ignoring bad formatting.
Integer overflow when calculating depth after multiple directory changes	Use an integer type large enough to avoid overflow, such as `long` in languages like Java or C++ if necessary or handle potential overflow by capping the depth at a maximum reasonable level.
Logs contain a very long string representing a child folder name	The string processing within the loop should handle extremely long strings efficiently without causing a buffer overflow or significant performance degradation.

Empty input log array

How to Handle:

Return 0, as no moves are made, implying we are already at the main folder.

Logs contain only './' operations

How to Handle:

Return 0, as './' operations do not change the current folder, so we remain at the main folder.

Logs contain only '../' operations, resulting in going beyond the main folder

How to Handle:

Handle '../' operations by decrementing the depth only if depth > 0; otherwise, stay at depth 0.

Logs contain a mix of '../' and child folder operations leading back to the main folder

How to Handle:

The algorithm accurately tracks the folder depth, returning to 0 after navigating back up.

Logs contain extremely deep child folder nesting

How to Handle:

The integer representing depth should handle a very large positive value representing the nesting level.

Logs contain invalid operation strings (e.g., 'abc/')

How to Handle:

Treat any string that doesn't match '../', './', or 'x/' as a child folder move ('x/'), effectively ignoring bad formatting.

Integer overflow when calculating depth after multiple directory changes

How to Handle:

Use an integer type large enough to avoid overflow, such as `long` in languages like Java or C++ if necessary or handle potential overflow by capping the depth at a maximum reasonable level.

Logs contain a very long string representing a child folder name

How to Handle:

The string processing within the loop should handle extremely long strings efficiently without causing a buffer overflow or significant performance degradation.