CODE

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */

public class Solution {
  public bool IsValidBST(TreeNode root) {
    traverse(root);

	if (!valid) return false;
	return true;
  }

  bool valid = true;
  List<int> empty = new List<int>();
  List<int> traverse(TreeNode root)
  {
    if (!valid || root == null) return empty;

	List<int> left = traverse(root.left);
	List<int> right = traverse(root.right);

	foreach (var l in left)
	  if (l >= root.val)
		valid = false;
	if (!valid) return empty;
	foreach (var r in right)
	  if (r <= root.val)
		valid = false;
	if (!valid) return empty;
	List<int> descendants = new List<int>();
	descendants.Add(root.val);
	foreach (var l in left)
	  descendants.Add(l);
	foreach (var r in right)
	  descendants.Add(r);
	return descendants;
  }
}