Network Delay Time

A network has `n` nodes labelled `1..n`. Each `[u, v, w]` in `times` is a directed link taking `w` time units. A signal is sent from node `k`. Return the time until every node has received it, or `-1` if some node never does. **Example 1:** ``` Input: times = [[2, 1, 1], [2, 3, 1], [3, 4, 1]], n = 4, k = 2 Output: 2 ``` **Example 2:** ``` Input: times = [[1, 2, 1]], n = 2, k = 1 Output: 1 ``` **Example 3:** ``` Input: times = [[1, 2, 1]], n = 2, k = 2 Output: -1 ```