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class Solution {
public boolean isValidBST(TreeNode root) {
return isValidBST(root, null, null);
}
private boolean isValidBST(TreeNode node, Integer min, Integer max) {
if (node == null)
return true;
if ((min != null && node.val <= min) || (max != null && node.val >= max))
return false;
return isValidBST(node.left, min, node.val) && isValidBST(node.right, node.val, max);
}
}