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class Solution {
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  Set<Integer> set = new HashSet<>();
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  public boolean findTarget(TreeNode root, int k) {
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    if (root == null) {
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      return false;
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    }
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    if (set.contains(k - root.val)) {
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      return true;
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    }
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    set.add(root.val);
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    return findTarget(root.left, k) || findTarget(root.right, k);
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  }
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}