How do you solve First Day Where You Have Been in All the Rooms on LeetCode?

First Day Where You Have Been in All the Rooms (LeetCode #1997) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: The only way to reach room i+1 is by visiting room i an even number of times.

What is the brute force solution for First Day Where You Have Been in All the Rooms?

The brute force approach for First Day Where You Have Been in All the Rooms is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach simulates each day of visiting rooms based on the rules provided. We keep track of how many times each room has been visited and determine the next room to visit based on the current visit count. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve First Day Where You Have Been in All the Rooms?

First Day Where You Have Been in All the Rooms uses the following concepts: Array, Dynamic Programming. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for First Day Where You Have Been in All the Rooms?

Always explain your thought process while coding, especially when transitioning from brute force to optimal solutions. Consider edge cases, such as the smallest or largest possible inputs. Practice simulating the process on paper before coding to clarify your approach.

What are common mistakes when solving First Day Where You Have Been in All the Rooms?

Failing to account for the modulo operation when returning the day. Not properly checking if all rooms have been visited before returning.

#1997

First Day Where You Have Been in All the Rooms

Medium

ArrayDynamic ProgrammingHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

💡

Intuition

Time O(n)Space O(n)

The optimal solution leverages the observation that the room visiting sequence can be modeled as a cycle. By tracking the number of visits and the next room efficiently, we can determine the first day all rooms are visited without simulating each day.

⚙️

Algorithm

4 steps

1Step 1: Initialize an array to count visits and a variable for the current room.
2Step 2: Use a loop to simulate the room visits until all rooms are visited.
3Step 3: Use a set to track visited rooms and a counter for days.
4Step 4: Return the day when all rooms have been visited.

solution.py20 lines

1# Full working Python code
2
3def firstDayVisitedAllRooms(nextVisit):
4    n = len(nextVisit)
5    visit_count = [0] * n
6    visited_rooms = set()
7    day = 0
8    current_room = 0
9    while len(visited_rooms) < n:
10        visit_count[current_room] += 1
11        visited_rooms.add(current_room)
12        if visit_count[current_room] % 2 == 1:
13            current_room = nextVisit[current_room]
14        else:
15            current_room = (current_room + 1) % n
16        day += 1
17    return day % (10**9 + 7)
18
19# Example usage:
20print(firstDayVisitedAllRooms([0, 0]))  # Output: 2

ℹ

Complexity note: The optimal solution runs in O(n) time because we only loop through the rooms a limited number of times, and we use a set to track visited rooms, which allows for efficient checks.

1The only way to reach room i+1 is by visiting room i an even number of times.
2Tracking the number of visits allows us to determine the next room efficiently.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.