🧠 Solving LeetCode Until I Become Top 1% — Day `48`

🔹 Problem: 1233. Remove Sub-Folders from the Filesystem

Difficulty: #Medium
Tags: #String, #Sorting

📝 Problem Summary

You’re given a list of folder paths (e.g., ["/a","/a/b","/c/d","/c/d/e","/c/f"]).
The task is to remove all sub-folders, keeping only the top-level parent folders.

A folder f1 is a sub-folder of f2 if f1 starts with f2 + "/".

Return a list of folders excluding all sub-folders.

🧠 My Thought Process

Brute Force Idea:
For each folder, check if it's a subfolder of any other folder in the list. That would be O(n²) comparisons. Not scalable.
Optimized Strategy:
Sort the folders lexicographically. That way, any subfolder will immediately follow its parent.
- Initialize the result list with the first folder.
- For every folder from index 1 onward:
- If it doesn’t start with the last added folder + '/', it's a top-level folder → add it to result.
Algorithm Used:

Sorting + Prefix Check

⚙️ Code Implementation (Python)

class Solution:
    def removeSubfolders(self, folder: List[str]) -> List[str]:
        folder.sort()  # Lexicographically sort paths
        res = [folder[0]]  # First one is always a root

        for f in folder[1:]:
            prev = res[-1]
            # Check if `f` is NOT a subfolder of the last added root
            if not (f.startswith(prev) and len(f) > len(prev) and f[len(prev)] == "/"):
                res.append(f)

        return res

⏱️ Time & Space Complexity

Time: O(n log n) — Sorting takes n log n, rest is linear scan
Space: O(n) — Result list

🧩 Key Takeaways

✅ Learned how lexicographic sorting simplifies hierarchical string problems.
💡 Tricky part was making sure we don't falsely match /a/bc as subfolder of /a.
💭 Will look out for prefix-matching problems where sorted input helps reduce complexity.

🔁 Reflection (Self-Check)

[x] Could I solve this without help?
[x] Did I write code from scratch?
[x] Did I understand why it works?
[x] Will I be able to recall this in a week?

🚀 Progress Tracker

Metric	Value
Day	`48`
Total Problems Solved	`389`
Confidence Today	😃
Leetcode Rating	`1572`

DEV Community