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590. N-ary Tree Postorder Traversal 👍

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  vector<int> postorder(Node* root) {
    if (root == nullptr)
      return {};

    vector<int> ans;
    stack<Node*> stack{{root}};

    while (!stack.empty()) {
      root = stack.top(), stack.pop();
      ans.push_back(root->val);
      for (Node* child : root->children)
        stack.push(child);
    }

    ranges::reverse(ans);
    return ans;
  }
};

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class Solution {
  public List<Integer> postorder(Node root) {
    if (root == null)
      return new ArrayList<>();

    List<Integer> ans = new ArrayList<>();
    Deque<Node> stack = new ArrayDeque<>();
    stack.push(root);

    while (!stack.isEmpty()) {
      root = stack.pop();
      ans.add(root.val);
      for (Node child : root.children)
        stack.push(child);
    }

    Collections.reverse(ans);
    return ans;
  }
}