Insufficient Nodes in Root to Leaf Paths LeetCode solution in Java

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class Solution {
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  public TreeNode helper(TreeNode root, int limit, int sumTillNow) {
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    if (root == null) return null;
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    if (root.left == null && root.right == null) return root.val + sumTillNow < limit ? null : root;
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    root.left = helper(root.left, limit, sumTillNow + root.val);
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    root.right = helper(root.right, limit, sumTillNow + root.val);
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    return root.left == null && root.right == null ? null : root;
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  }
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  public TreeNode sufficientSubset(TreeNode root, int limit) {
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    return helper(root, limit, 0);
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  }
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}