590. N-ary Tree Postorder Traversal

Problem

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, 10^4].

• 0 <= Node.val <= 10^4

• 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?

Solution (Java)

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    private List<Integer> ans;

    public List<Integer> postorder(Node root) {
        ans = new ArrayList<>();
        recursion(root);
        if (root != null) {
            ans.add(root.val);
        }
        return ans;
    }

    private void recursion(Node root) {
        if (root == null) {
            return;
        }
        for (Node child : root.children) {
            recursion(child);
            ans.add(child.val);
        }
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).