Count Good Nodes in Binary Tree LeetCode solution in Java

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class Solution {
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  int ans = 0;
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  public int goodNodes(TreeNode root) {
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    if (root == null) return 0;
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    dfs(root, root.val);
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    return ans;
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  }
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  void dfs(TreeNode root, int mx) {
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    if (root == null) return;
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    mx = Math.max(mx, root.val);
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    if (mx <= root.val) ans++;
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    dfs(root.left, mx);
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    dfs(root.right, mx);
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  }
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}