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class Solution {
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  int ans = Integer.MAX_VALUE;
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  boolean x = true;
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  public int findSecondMinimumValue(TreeNode root) {
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    go(root);
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    return x ? -1 : ans;
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  }
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  private void go(TreeNode root) {
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    if (root == null) return;
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    if (root.left != null) {
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      if (root.left.val == root.val) go(root.left);
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      else {
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        x = false;
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        ans = Math.min(ans, root.left.val);
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      }
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    }
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    if (root.right != null) {
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      if (root.right.val == root.val) go(root.right);
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      else {
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        x = false;
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        ans = Math.min(ans, root.right.val);
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      }
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    }
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  }
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}