Binary Search Tree to Greater Sum Tree LeetCode solution in Java

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class Solution {
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  int sum = 0;
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  public TreeNode bstToGst(TreeNode root) {
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    if (root != null) {
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      bstToGst(root.right);
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      sum += root.val;
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      root.val = sum;
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      bstToGst(root.left);
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    }
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    return root;
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  }
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}