Flip Binary Tree To Match Preorder Traversal LeetCode solution in Java

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class Solution {
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  int vix = 0;
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  List<Integer> ans = new ArrayList<>();
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  private void dfs(TreeNode node, int[] V) {
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    if (node == null || (ans.size() != 0 && ans.get(0) == -1)) return;
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    if (node.val != V[vix++]) ans = new ArrayList<Integer>(Arrays.asList(-1));
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    else if (node.left != null && node.left.val != V[vix]) {
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      ans.add(node.val);
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      dfs(node.right, V);
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      dfs(node.left, V);
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    } else {
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      dfs(node.left, V);
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      dfs(node.right, V);
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    }
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  }
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  public List<Integer> flipMatchVoyage(TreeNode root, int[] V) {
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    dfs(root, V);
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    return ans;
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  }
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}