March LeetCoding Challenge 2021 — Day 19: Keys and Rooms

Today, we will solve the 19th problem of the March LeetCoding Challenge.

Problem Statement

There are N rooms and you start in room 0. Each room has a distinct number in 0, 1, 2, ..., N-1, and each room may have some keys to access the next room.

Formally, each room i has a list of keys rooms[i], and each key rooms[i][j] is an integer in [0, 1, ..., N-1] where N = rooms.length. A key rooms[i][j] = v opens the room with number v.

Initially, all the rooms start locked (except for room 0).

You can walk back and forth between rooms freely.

Return true if and only if you can enter every room.

Example 1:

**Input: **[[1],[2],[3],[]]
**Output: **true
**Explanation:  **
We start in room 0, and pick up key 1.
We then go to room 1, and pick up key 2.
We then go to room 2, and pick up key 3.
We then go to room 3.  Since we were able to go to every room, we return true.

Example 2:

**Input: **[[1,3],[3,0,1],[2],[0]]
**Output: **false
**Explanation: **We can't enter the room with number 2.

Note:

1. 1 <= rooms.length <= 1000

2. 0 <= rooms[i].length <= 1000

3. The number of keys in all rooms combined is at most 3000.

Solution

In this problem, we have to find whether we can visit all the rooms. Initially, the 0-th room is opened, and at every index, we have the keys to go to other rooms. So, we can visualize this problem as a graph-based problem.

Once we enter the 0-th room, we will check for keys in that room. Once we find a key, we directly move to that room and repeat the process. We keep the count of visited rooms in a HashSet. If the size of the HashSet is equal to the number of rooms, we return true else false. We are using breath-first-search to solve this problem.

The code can be found here

sksaikia / LeetCode

LeetCode

I have been solving Leetcode problems for around a year or so. Suddenly I have developed a passion of writing tutorials for these problems. I am starting with Leetcode problem, in future I will try to make tutorials on Spring,Android,Java,Algorithms and many more.

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The following table contains all the problems with their respective solution tutorials. I will try to add more articles to it soon.

Problem Name	Problem No	Article Links
Contains Duplicate	217	https://sourav-saikia.medium.com/leetcode-217-contains-duplicate-solution-36f48793bce0
Jewels and Stones	771	https://sourav-saikia.medium.com/leetcode-771-jewels-and-stones-e35a368fc37

March LeetCoding Challenge

Problem Name	Problem No	Article Links
Distribute Candies	575	https://medium.com/dev-genius/march-leetcoding-challenge-2021-problem-1-distribute-candies-f37f66ea7ee9
Set Mismatch	645	https://sourav-saikia.medium.com/march-leetcoding-challenge-2021-day-2-set-mismatch-4abd5ee491c9
Missing Number	268	https://sourav-saikia.medium.com/march-leetcoding-challenge-2021-day-3-missing-number-ae8ee45a58cb
Intersection of Two Linked Lists	160	https://sourav-saikia.medium.com/march-leetcoding-challenge-2021-day-4-intersection-of-two-linked-lists-a775449b5563
Average of Levels in Binary Tree	637	https://medium.com/dev-genius/march-leetcoding-challenge-2021-day-5-average-of-levels-in-binary-tree-ac886b2b42e8
Short Encoding of Words	820	https://sourav-saikia.medium.com/march-leetcoding-challenge-2021-day-6-short-encoding-of-words-7fed4bfae557
Remove Palindromic Subsequences	1332

…

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Check out my other posts on March LeetCoding Challenge 2021.

1. March LeetCoding Challenge — Day 1 — Distribute Candies

2. March LeetCoding Challenge — Day 2 — Set Mismatch

3. March LeetCoding Challenge — Day 3 — Missing Number

4. March LeetCoding Challenge — Day 4 — Intersection of Two Linked Lists

5. March LeetCoding Challenge — Day 5 — Average of Levels in Binary Tree

6. March LeetCoding Challenge — Day 6 — Short Encoding of Words

7. March LeetCoding Challenge — Day 7 — Design HashMap

8. March LeetCoding Challenge — Day 8 — Remove Palindromic Subsequences

9. March LeetCoding Challenge — Day 10 — Integer to Roman

10. March LeetCoding Challenge — Day 12 — Check If a String Contains All Binary Codes of Size K

11. March LeetCoding Challenge — Day 14 — Swapping Nodes in a Linked List

12. March LeetCoding Challenge — Day 15 — Encode and Decode TinyURL

13. March LeetCoding Challenge — Day 18 — Wiggle Subsequence