Network Delay Time

Merge K Sorted Lists

Pacific Atlantic Water Flow

Compute how long a weighted directed network takes to receive a signal.

Edges `[u,v,w]` take `w` time. Return when every node receives a signal sent from `k`, or `-1` if any node is unreachable.

## Example
```json
Input: {"times":[[2,1,1],[2,3,1],[3,4,1]],"n":4,"k":2}
Output: 2
```

## Constraints
- 1 <= k <= n <= 100
- 1 <= times.length <= 6000
- 0 <= w <= 100

## Follow-up
When would Bellman-Ford be necessary instead of Dijkstra?

The key pattern is: Dijkstra settles nodes by earliest signal arrival. Identify the state and invariant before coding.

Recognize the Pattern

Advance one state transition at a time. Mark or update state before exploring dependent work.

Build the Algorithm

Test empty or minimal input, skewed shapes, duplicates where allowed, and impossible outcomes.