• Long.MAX_VALUE, Long.MIN_VALUE: or, will fail by the test case 2147483647;
  • BST: cannot equal;
  • dfs template: terminate condition first, then recursively call function, i.e. root==null, return and root.val >= mx || root.val <= mn, return
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class Solution {
    public boolean isValidBST(TreeNode root) {
        return valid(root, Long.MAX_VALUE, Long.MIN_VALUE);
    }
    
    public boolean valid(TreeNode root, long mx, long mn) {
        if (root == null) return true;
        if (root.val >= mx || root.val <= mn) return false;
        return valid(root.left, root.val, mn) && valid(root.right, mx, root.val);
    }
}