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

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