Crawler Log Folder

LeetCode 1720 | Difficulty: Easy

Easy

Problem Description

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 <= 10^3`

- `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.

Topics: Array, String, Stack

---

Approach

Stack

Use a stack (LIFO) to track elements that need future processing. Process elements when a "trigger" condition is met (e.g., finding a smaller/larger element). Monotonic stack maintains elements in sorted order for next greater/smaller element problems.

When to use

Matching brackets, next greater element, evaluating expressions, backtracking history.

String Processing

Consider character frequency counts, two-pointer approaches, or building strings efficiently. For pattern matching, think about KMP or rolling hash. For palindromes, expand from center or use DP.

When to use

Anagram detection, palindrome checking, string transformation, pattern matching.

---

Solutions

Solution 1: C# (Best: 92 ms)

Metric	Value
Runtime	92 ms
Memory	38.8 MB
Date	2022-01-18

Solution

public class Solution {
    public int MinOperations(string[] logs) {
        int count = 0;
        foreach(var log in logs)
        {
            
            if(log=="../") count = Math.Max(--count, 0);
            else if(log!="./") {count++;}
        }
        return count;
    }
}

---

Complexity Analysis

Approach	Time	Space
Stack	$O(n)$	$O(n)$

---

Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• Think about what triggers a pop: is it finding a match, or finding a smaller/larger element?
• LeetCode provides 1 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: Simulate the process but don’t move the pointer beyond the main folder.