Binary Tree Level Order Traversal LeetCode solution in Java

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class Solution {
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  public List<List<Integer>> levelOrder(TreeNode root) {
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    List<List<Integer>> result = new ArrayList<>();
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    if (root == null) return result;
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    List<Integer> rootList = new ArrayList<>();
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    rootList.add(root.val);
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    result.add(rootList);
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    levelOrder(root, 1, result);
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    return result;
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  }
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  private void levelOrder(TreeNode root, int level, List<List<Integer>> result) {
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    if (root == null) return;
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    List<Integer> children = exploreChildren(root);
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    if (!children.isEmpty()) {
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      if (level < result.size()) result.get(level).addAll(children);
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      else result.add(children);
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    }
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    levelOrder(root.left, level + 1, result);
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    levelOrder(root.right, level + 1, result);
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  }
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  private List<Integer> exploreChildren(TreeNode root) {
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    List<Integer> children = new ArrayList<>();
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    if (root.left != null) children.add(root.left.val);
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    if (root.right != null) children.add(root.right.val);
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    return children;
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  }
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}