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RAII Release Order

Work out the order in which scoped resources are released when their scopes end, as RAII guarantees.

Cosa imparerai

  • Release resources when a scope ends
  • Destroy objects in reverse creation order
  • Model nested scopes with a stack

Problema

Mostra traduzione

In C++, RAII ties a resource to an object's lifetime: the resource is released automatically when the object goes out of scope. Objects in the same scope are destroyed in reverse order of creation.

Process the events in order:

  • {: enter a new inner scope.
  • }: leave the current scope; every resource acquired in it is released, newest first.
  • acquire <name>: create a resource in the current scope.

When the events run out, the remaining open scopes end from the innermost to the outermost (the program's outer scope included), releasing their resources the same way.

Implement raii_release_order(events).

Input

events: The events. Braces are balanced or left open, never closed without being opened.

Output

Return the resource names in the order they are released.

Vincoli

0 ≤ number of events ≤ 100,000

Names have 1 to 20 characters without spaces

Esempi

Esempio 1
Input:
events = ["acquire db", "{", "acquire file", "acquire lock", "}", "acquire log"]

Output atteso:
expected = ["lock", "file", "log", "db"]
Input:
std::vector<std::string> events = {"acquire db", "{", "acquire file", "acquire lock", "}", "acquire log"};

Output atteso:
std::vector<std::string> expected = {"lock", "file", "log", "db"};
Input:
string[] events = new string[] {"acquire db", "{", "acquire file", "acquire lock", "}", "acquire log"};

Output atteso:
string[] expected = new string[] {"lock", "file", "log", "db"};

Spiegazione

Leaving the inner scope releases lock then file (newest first). At the end the outer scope releases log then db: ["lock", "file", "log", "db"].

Esempio 2
Input:
events = ["{", "acquire a", "{", "acquire b"]

Output atteso:
expected = ["b", "a"]
Input:
std::vector<std::string> events = {"{", "acquire a", "{", "acquire b"};

Output atteso:
std::vector<std::string> expected = {"b", "a"};
Input:
string[] events = new string[] {"{", "acquire a", "{", "acquire b"};

Output atteso:
string[] expected = new string[] {"b", "a"};

Spiegazione

Two scopes are still open at the end: the innermost releases b, then the next releases a: ["b", "a"].

Come affrontarlo

Prova prima da solo, poi apri un suggerimento alla volta.

Suggerimento 1

Keep a stack of scopes, each holding its resources in order.

Suggerimento 2

Closing a scope releases its resources from newest to oldest.

Suggerimento 3

At the end, close every scope that is still open, innermost first.

Complessità attesa

O(number of events) time, O(number of events) extra space

Riferimenti

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