N-ary Tree Level Order Traversal LeetCode solution in Java

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class Solution {
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  public List<List<Integer>> result = new ArrayList();
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  public List<List<Integer>> levelOrder(Node root) {
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    if (root == null) return result;
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    helper(root, 0);
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    return result;
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  }
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  private void helper(Node node, int level) {
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    if (result.size() <= level) {
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      result.add(new ArrayList());
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    }
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    result.get(level).add(node.val);
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    for (Node child : node.children) {
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      helper(child, level + 1);
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    }
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  }
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}