Leetcode #588: Design In-Memory File System
In this guide, we solve Leetcode #588 Design In-Memory File System in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Design a data structure that simulates an in-memory file system. Implement the FileSystem class: FileSystem() Initializes the object of the system.
Quick Facts
- Difficulty: Hard
- Premium: Yes
- Tags: Design, Trie, Hash Table, String, Sorting
Intuition
Fast membership checks and value lookups are the heart of this problem, which makes a hash map the natural choice.
By storing what we have already seen (or counts/indexes), we can answer the question in one pass without backtracking.
Approach
Scan the input once, using the map to detect when the condition is satisfied and to update state as you go.
This keeps the solution linear while remaining easy to explain in an interview setting.
Steps:
- Initialize a hash map for seen items or counts.
- Iterate through the input, querying/updating the map.
- Return the first valid result or the final computed value.
Example
Input
["FileSystem", "ls", "mkdir", "addContentToFile", "ls", "readContentFromFile"]
[[], ["/"], ["/a/b/c"], ["/a/b/c/d", "hello"], ["/"], ["/a/b/c/d"]]
Output
[null, [], null, null, ["a"], "hello"]
Explanation
FileSystem fileSystem = new FileSystem();
fileSystem.ls("/"); // return []
fileSystem.mkdir("/a/b/c");
fileSystem.addContentToFile("/a/b/c/d", "hello");
fileSystem.ls("/"); // return ["a"]
fileSystem.readContentFromFile("/a/b/c/d"); // return "hello"
Python Solution
class Trie:
def __init__(self):
self.name = None
self.isFile = False
self.content = []
self.children = {}
def insert(self, path, isFile):
node = self
ps = path.split('/')
for p in ps[1:]:
if p not in node.children:
node.children[p] = Trie()
node = node.children[p]
node.isFile = isFile
if isFile:
node.name = ps[-1]
return node
def search(self, path):
node = self
if path == '/':
return node
ps = path.split('/')
for p in ps[1:]:
if p not in node.children:
return None
node = node.children[p]
return node
class FileSystem:
def __init__(self):
self.root = Trie()
def ls(self, path: str) -> List[str]:
node = self.root.search(path)
if node is None:
return []
if node.isFile:
return [node.name]
return sorted(node.children.keys())
def mkdir(self, path: str) -> None:
self.root.insert(path, False)
def addContentToFile(self, filePath: str, content: str) -> None:
node = self.root.insert(filePath, True)
node.content.append(content)
def readContentFromFile(self, filePath: str) -> str:
node = self.root.search(filePath)
return ''.join(node.content)
# Your FileSystem object will be instantiated and called as such:
# obj = FileSystem()
# param_1 = obj.ls(path)
# obj.mkdir(path)
# obj.addContentToFile(filePath,content)
# param_4 = obj.readContentFromFile(filePath)
Complexity
The time complexity is O(n). The space complexity is O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.