/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    Stack<TreeNode> S = new Stack<TreeNode>();

        List<List<TreeNode>> list = new List<List<TreeNode>>();

        List<TreeNode> left = new List<TreeNode>();

        private void postNode(TreeNode node)
        {
            if (node != null)
            {
                S.Push(node);
                if (node.left != null)
                {
                    if (node.left.left == null && node.left.right == null)
                    {
                        left.Add(node.left);
                    }
                    postNode(node.left);
                }
                if (node.right != null)
                {
                    postNode(node.right);
                }
                if (node.left == null && node.right == null)
                {
                    list.Add(S.ToList());
                }

                S.Pop();
            }
        }

        public int FindBottomLeftValue(TreeNode root)
        {
            postNode(root);

            list = list.OrderByDescending(x => x.Count).ToList();

            var result = root.val;
            foreach (var l in list)
            {
                result = l[0].val;
                break;
            }
            return result;
        }
}