https://leetcode.com/problems/maximal-network-rank/description/
1615. Maximal Network Rank
給你一個 n 表示城市的數量，一個陣列 roads[][] ，roads[i][j] 表示
城市 i 存在直達城市 j 的雙向道路，Network Rank 被定義為任意兩城市
的所有直連道路總和（若兩城市間存在道路則只算一個），求出最大
Network Rank。
Example 1:
https://assets.leetcode.com/uploads/2020/09/21/ex1.png
Input: n = 4, roads = [[0,1],[0,3],[1,2],[1,3]]
Output: 4
Explanation: The network rank of cities 0 and 1 is 4 as there are 4 roads
that are connected to either 0 or 1. The road between 0 and 1 is only counted
once.
Example 2:
https://assets.leetcode.com/uploads/2020/09/21/ex2.png
Input: n = 5, roads = [[0,1],[0,3],[1,2],[1,3],[2,3],[2,4]]
Output: 5
Explanation: There are 5 roads that are connected to cities 1 or 2.
Example 3:
Input: n = 8, roads = [[0,1],[1,2],[2,3],[2,4],[5,6],[5,7]]
Output: 5
Explanation: The network rank of 2 and 5 is 5. Notice that all the cities do
not have to be connected.
思路：
1.按照題意，一個 Network Rank 是兩個城市的道路總和，我們把城市看成點道路看成
線，就是要我們求任意兩個點的最大 degree 和。
2.我們用一個陣列記住每個點向外的道路有幾個，一個Set紀錄任意兩點是否連通。
3.最後把每個城市兩兩配對，將道路數量相加（若兩城市相連減一），並取最大的 rank
即可。
Java Code：