Simplify Path

LeetCode 71 | Difficulty: Medium

Medium

Problem Description

You are given an absolute path for a Unix-style file system, which always begins with a slash '/'. Your task is to transform this absolute path into its simplified canonical path.

The rules of a Unix-style file system are as follows:

- A single period `'.'` represents the current directory.

- A double period `'..'` represents the previous/parent directory.

- Multiple consecutive slashes such as `'//'` and `'///'` are treated as a single slash `'/'`.

- Any sequence of periods that does **not match** the rules above should be treated as a **valid directory or** **file ****name**. For example, `'...' `and `'....'` are valid directory or file names.

The simplified canonical path should follow these rules:

- The path must start with a single slash `'/'`.

- Directories within the path must be separated by exactly one slash `'/'`.

- The path must not end with a slash `'/'`, unless it is the root directory.

- The path must not have any single or double periods (`'.'` and `'..'`) used to denote current or parent directories.

Return the simplified canonical path.

Example 1:

Input: path = "/home/"

Output: "/home"

Explanation:

The trailing slash should be removed.

Example 2:

Input: path = "/home//foo/"

Output: "/home/foo"

Explanation:

Multiple consecutive slashes are replaced by a single one.

Example 3:

Input: path = "/home/user/Documents/../Pictures"

Output: "/home/user/Pictures"

Explanation:

A double period ".." refers to the directory up a level (the parent directory).

Example 4:

Input: path = "/../"

Output: "/"

Explanation:

Going one level up from the root directory is not possible.

Example 5:

Input: path = "/.../a/../b/c/../d/./"

Output: "/.../b/d"

Explanation:

"..." is a valid name for a directory in this problem.

Constraints:

- `1 <= path.length <= 3000`

- `path` consists of English letters, digits, period `'.'`, slash `'/'` or `'_'`.

- `path` is a valid absolute Unix path.

Topics: String, Stack

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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.

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Solutions

Solution 1: C# (Best: 148 ms)

Metric	Value
Runtime	148 ms
Memory	36.4 MB
Date	2022-01-14

Solution

public class Solution {
    public string SimplifyPath(string path) {
        string[] arr = path.Split(new char[] {'/'}, StringSplitOptions.RemoveEmptyEntries);
    Stack<string> s = new Stack<string>();
    foreach (var entry in arr)
    {
        if(entry == ".") continue;
        if(entry == "..")
        {
            if(s.Count > 0) s.Pop();
        }
        else
        {
            s.Push(entry);
        }
    }
    
    return "/"+ String.Join("/",s.Reverse().ToList());

    }
}

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Complexity Analysis

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

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Interview Tips

Key Points

• Discuss the brute force approach first, then optimize. Explain your thought process.
• Think about what triggers a pop: is it finding a match, or finding a smaller/larger element?