Binary Tree Preorder Traversal LeetCode solution in Java

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class Solution {
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  public List<Integer> preorderTraversal(TreeNode root) {
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    List<Integer> result = new ArrayList<>();
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    preorderTraversal2(root, result);
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    return result;
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  }
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  public List<Integer> preorderTraversal2(TreeNode root, List<Integer> result) {
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    if (root != null) {
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      result.add(root.val);
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      if (root.left != null) {
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        preorderTraversal2(root.left, result);
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      }
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      if (root.right != null) {
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        preorderTraversal2(root.right, result);
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      }
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    }
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    return result;
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  }
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}