N-ary Tree Postorder Traversal

`Tree`

Easy

Given an n-ary tree, return thepostordertraversal of its nodes' values.

For example, given a`3-ary`tree:

Return its postorder traversal as:`[5,6,3,2,4,1]`.

Note:

Recursive solution is trivial, could you do it iteratively?

Solution

Iterative - with stack

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

public Node() {}

public Node(int _val,List<Node> _children) {
val = _val;
children = _children;
}
};
*/
class Solution {
public List<Integer> postorder(Node root) {
if (root == null) {
return res;
}
stack.push(root);

while (!stack.isEmpty()) {
Node node = stack.pop();

if (node.children != null) {
for (Node c: node.children) {
if (c != null) {
stack.push(c);
}
}
}
}

return res;
}
}``````

Complexity Analysis

• Time complexity : we visit each node exactly once, thus the time complexity is O(N), where N is the number of nodes, _i.e._ the size of tree.

• Space complexity : depending on the tree structure, we could keep up to the entire tree, therefore, the space complexity is O(N).

Recursive

``````class Solution {
public List<Integer> postorder(Node root) {
if (root == null) {
return res;
}

postorderHelper(root, res);

return res;
}

private void postorderHelper(Node root, List<Integer> res) {
if (root == null) {
return;
}
if (root.children != null) {
for (Node c : root.children) {
postorderHelper(c, res);
}
}