Input: times = [[2,1,1],[2,3,1],[3,4,1]], n = 4, k = 2
Output: 2
Explanation:  2 -> 1 (1 unit)  and 2 -> 3 -> 4 ( 2 unit )
The last node (4) gets the signal at time 2.

Input: times = [[1,2,1]], n = 2, k = 1
Output: 1
Explanation:  Direct edge 1 -> 2 delivers signal in 1 unit of time.

Disclaimer: Here is the practice link to help you assess your knowledge better. It's highly recommend trying to solve it before looking at the solution.

Approach

Algorithm

We want to find the time it takes for a signal to reach all nodes from a starting point K. This is equivalent to finding the shortest time (minimum delay) from K to every other node, which is exactly what Dijkstra’s algorithm does for graphs with non-negative edge weights.
It works by greedily selecting the nearest unvisited node, then relaxing its neighbors.

Create a graph using an adjacency list to store edges and their weights.
Use a distance array to store the shortest time to reach each node, initialized to infinity except the source node, which is 0.
Use a min-heap (priority queue) to always pick the node with the current shortest time.
For each node picked from the heap, check all its neighbors. If reaching a neighbor through the current node gives a shorter time, update the neighbor’s time and push it into the heap.
Repeat until the heap is empty. At the end, check the maximum value in the distance array.
If any node's time is still infinity, return -1 (unreachable). Otherwise, return the maximum time.