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class Solution {
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  boolean ans = true;
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  int firstVal = 0;
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  public boolean isUnivalTree(TreeNode root) {
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    if (root == null) return ans;
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    firstVal = root.val;
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    traversal(root);
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    return ans;
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  }
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  private void traversal(TreeNode root) {
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    if (root == null) return;
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    if (root.val != firstVal) ans = false;
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    traversal(root.left);
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    traversal(root.right);
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  }
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}