0590. N-ary Tree Postorder Traversal

https://leetcode.com/problems/n-ary-tree-postorder-traversal

Description

Given the root of an n-ary tree, return the postorder traversal of its nodes' values.

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

Example 1:

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

Example 2:

**Input:** root = [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:** [2,6,14,11,7,3,12,8,4,13,9,10,5,1]

Constraints:

• The number of nodes in the tree is in the range [0, 104].

• 0 <= Node.val <= 104

• The height of the n-ary tree is less than or equal to 1000.

Follow up: Recursive solution is trivial, could you do it iteratively?

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