N-ary Tree Postorder Traversal LeetCode solution in Java

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class Solution {
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  List<Integer> result = new ArrayList<>();
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  public List<Integer> postorder(Node root) {
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    addNodes(root);
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    return result;
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  }
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  void addNodes(Node root) {
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    if (root == null) return;
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    for (Node child : root.children) addNodes(child);
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    result.add(root.val);
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  }
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}