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1719. Number Of Ways To Reconstruct A Tree

You are given an array pairs, where pairs[i] = [xi, yi], and:

There are no duplicates.
xi < yi
Let ways be the number of rooted trees that satisfy the following conditions:

The tree consists of nodes whose values appeared in pairs.
A pair [xi, yi] exists in pairs if and only if xi is an ancestor of yi or yi is an ancestor of xi.
Note: the tree does not have to be a binary tree.
Two ways are considered to be different if there is at least one node that has different parents in both ways.

Return:

0 if ways == 0
1 if ways == 1
2 if ways > 1
A rooted tree is a tree that has a single root node, and all edges are oriented to be outgoing from the root.

An ancestor of a node is any node on the path from the root to that node (excluding the node itself). The root has no ancestors.

解题重点：
1. 题目的意思是，pairs如果能构成树，那pairs的结对和树拆散后要一致，不能有多余的节点。

2. 通过假设pairs能构成树，找到一些规律，或者推论。节点总数为n，节点 x 的祖先和后代的节点集合表示 adj[x]

3. 根结点为root, 有adj[root]的数量为n-1

4. 如果x为y的祖先，则有len(adj[x]) >= len(adj[y])，adj[y]是adj[x]的子集。

5. 如果x为y的祖先，且adj[xi]=adj[yi]，则此时构成树的方案数一定不是唯一的。

6.找父节点，依次跟父节点对比。父节点：节点 x 满足 len(adj[x])最小且 len(adj[x])>= len(ajd[node])。判断子集关系，判断节点数量，相等则有多种可能。