Add One Row to Tree LeetCode solution in Java

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class Solution {
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  public TreeNode addOneRow(TreeNode root, int v, int d) {
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    if (d == 1) return new TreeNode(v, root, null);
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    else if (d == 2) {
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      root.left = new TreeNode(v, root.left, null);
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      root.right = new TreeNode(v, null, root.right);
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    } else {
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      if (root.left != null) addOneRow(root.left, v, d - 1);
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      if (root.right != null) addOneRow(root.right, v, d - 1);
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    }
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    return root;
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  }
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}