How do you approach Keys and Rooms?

DFS/BFS from room 0, collecting keys as you go.

What are common mistakes on Keys and Rooms?

Building the adjacency list in one direction on an undirected graph. Assuming the graph is connected. Most inputs allow several components, and one traversal from node 0 misses the rest. Ignoring self-loops and parallel edges when the constraints allow them.

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Keys and Rooms

MediumAlgorithms

Description

There are n rooms labeled 0 to n - 1. You start in room 0. Each room i contains a list of keys rooms[i], each opening another room.

Return true if you can visit all the rooms, otherwise false.

Example 1

Input: rooms = [[1],[2],[3],[]]

Output: true

Explanation: 0 -> 1 -> 2 -> 3.

Example 2

Input: rooms = [[1,3],[3,0,1],[2],[0]]

Output: false

Explanation: Room 2 is never reachable.

Constraints

- n == rooms.length - 2 <= n <= 1000 - rooms[i] contains distinct values in [0, n - 1]

Keys and Rooms: approach, complexity and pitfalls

A medium graphs problem, graded against 7 test cases (5 of them hidden).

The pattern: Graphs

Nodes and edges - traversal, connectivity, cycles, and shortest paths.

Reach for it when you see: Explicit edges, a grid treated as a graph, or dependencies between items.

If you are stuck

Show the 2-step hint ladder

DFS/BFS from room 0, collecting keys as you go.
Count how many distinct rooms you can reach and compare to n.

The optimal approach

Show the approach: The approach

Graph traversal. Treat keys as directed edges and DFS/BFS from room 0, marking each newly entered room visited. You can visit all rooms iff the count of visited rooms equals `n`.

Time: O(n + total keys). Space: O(n).

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Empty input. At least one case passes an empty collection. Make sure your loop bounds and any "first element" read survive a length of zero.
Single element. A one-element case is included. Two-pointer and window solutions frequently return the wrong thing when the two indices start out equal.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Graphs mistakes

These apply to the pattern as a whole, not just this problem.

Building the adjacency list in one direction on an undirected graph.
Assuming the graph is connected. Most inputs allow several components, and one traversal from node 0 misses the rest.
Ignoring self-loops and parallel edges when the constraints allow them.