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 TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    traverse(root);
	for (int i = 0; i < dpn.Count; i++)
	  if (dpn[i].Contains(p.val) && dpn[i].Contains(q.val))
		return nodes[i];
      return root;
    }

    List<List<int>> dpn = new List<List<int>>();
    List<TreeNode> nodes = new List<TreeNode>();
    List<int> empty = new List<int>();

  List<int> traverse(TreeNode root)
  {
    if (root == null) return empty;
    List<int> left = traverse(root.left);
    List<int> right = traverse(root.right);
    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);
    List<int> descCopy = new List<int>(descendants);
    dpn.Add(descCopy);
    nodes.Add(root);
    return descendants;
  }
}