1506. Find Root of N-Ary Tree

Given all the nodes of an N-ary tree as an array  Node[] tree where each node has a unique value.

Find and return the root of the N-ary tree.

Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Follow up: Can you find the root of the tree with O(1) additional memory space?

Notes:

1. The following input is only given to testing purposes.
2. You will receive as input a list of all nodes of the n-ary tree in any order.

Example 1:

Input: [1,null,3,2,4,null,5,6]
Output: [1,null,3,2,4,null,5,6]

Example 2:

Input: [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]

Constraints:

• The total number of nodes is between [1, 5*10^4].
• Each node has a unique value.