Crawler Log Folder - Solution & Explanation

Problem Overview: You are given a list of folder navigation logs such as "../", "./", or "x/". Each log represents moving up a directory, staying in the current folder, or entering a subfolder. The goal is to compute the minimum number of operations needed to return to the main folder after processing all logs.

The commands behave exactly like terminal navigation. "../" moves one level up (unless you are already at the root), "./" keeps you in the same directory, and any other string ending with "/" means entering a new folder. The problem becomes a simple state tracking task where you simulate how deep you are in the folder structure.

Approach 1: Using Depth Counter (O(n) time, O(1) space)

This approach keeps a single integer variable representing the current folder depth. Iterate through the array of logs and update the depth based on each command. When encountering "../", decrease the depth only if it is greater than zero. Ignore "./" because it does not change the directory. For any other log (like "d1/"), increment the depth because you move into a subfolder.

The key insight is that the actual folder names do not matter. Only the direction of movement matters. Because the algorithm processes each log once and stores only an integer counter, it runs in O(n) time with O(1) extra space. This is the simplest and most efficient solution and is typically what interviewers expect.

Approach 2: Using Stack for Folder Simulation (O(n) time, O(n) space)

This method simulates the directory structure using a stack. Traverse the logs one by one. When the command is a folder name like "x/", push it onto the stack. When the command is "../", pop from the stack if it is not empty. Ignore "./" since it keeps you in the same folder.

After processing all logs, the number of elements remaining in the stack represents how many directories deep you are from the root. That value equals the minimum operations needed to return to the main folder. This approach models the navigation process explicitly and mirrors how real file systems manage paths.

The stack-based solution also runs in O(n) time because each log is processed once. However, it uses O(n) space in the worst case if every log moves into a new folder. The logic relies heavily on operations typical in string processing and stack manipulation.

Recommended for interviews: The depth counter solution is usually preferred. It shows that you recognized the problem only tracks folder depth rather than actual paths. The stack approach is still valuable because it demonstrates understanding of directory simulation and stack-based state management.