Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to check whether these edges make up a valid tree.

For example:

Given n = 5 and edges = [[0, 1], [0, 2], [0, 3], [1, 4]], return true.

Given n = 5 and edges = [[0, 1], [1, 2], [2, 3], [1, 3], [1, 4]]

Hint:

1. Given n = 5 and edges = [[0, 1], [1, 2], [3, 4]], what should your return? Is this case a valid tree?
2. According to the definition of tree on Wikipedia: “a tree is an undirected graph in which any two vertices are connected by exactly one path. In other words, any connected graph without simple cycles is a tree.”

Note: you can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

給定n個標記為0到n-1的節點和一個無向邊列表（每個邊都是一對節點），編寫一個函數來檢查這些邊是否構成有效的樹。

例如：

如果n=5且邊數為[[0，1]，[0，2]，[0，3]，[1，4]]，則返回true。

如果n=5且邊數為[[0，1]，[1，2]，[2，3]，[1，3]，[1，4]]，則返回false。

提示：

考慮到n=5和edges=[[0，1]，[1，2]，[3，4]]，你應該返回什麽？這是一個有效的樹嗎？

根據維基百科上樹的定義：“樹是一個無向圖，其中任何兩個頂點都由一條路徑連接。換句話說，任何沒有簡單循環的連通圖都是一棵樹。”

註意：可以假定邊中不會出現重復的邊。因為所有邊都是無向的，[0，1]與[1，0]相同，因此不會在邊中一起出現。

BFS

 1 class Solution {
 2     func validTree(_ n:Int,_ edges:[[Int]]) -> Bool{
 3         var g:[Set<Int>] = [Set<Int>](repeating:Set<Int>(),count:n)
 4         var s:Set<Int> = [0]
 5         var q:[Int] = [0]
 6         for a in edges
 7         {
 8             g[a[0]].insert(a[1]);
 9             g[a[1]].insert(a[0]);
10         }
11         while(!q.isEmpty)
12         {
13             var t:Int = q.first!
14             q.removeFirst()
15             for a in g[t]
16             {
17                 if s.contains(a)
18                 {
19                     return false
20                 }
21                 s.insert(a)
22                 q.append(a)
23                 g[a].remove(t)
24             }
25         }
26          return s.count == n
27     }
28 }

[Swift]LeetCode261.圖驗證樹 $ Graph Valid Tree