leetcode988

 public class Solution
     {
         private Stack<string> ST = new Stack<string>();
         private string SmallestStr = String.Empty;
         private string[] Ary = new string[] { "a","b","c","d","e","f","g",
         "h","i","j","k","l","m","n","o","p","q","r","s","t",
             "u","v","w","x","y","z"};
         private void SearchTree(TreeNode root)
         {
             if(root!=null)
             {
                 var index = root.val;
                 var str = Ary[index];
                 ST.Push(str);
 
                 if(root.left!=null)
                 {
                     SearchTree(root.left);
                 }
 
                 if(root.right!=null)
                 {
                     SearchTree(root.right);
                 }
                 if(root.left==null && root.right==null)
                 {
                     //find a leaf node
                     var a1 = ST.ToArray();
                     StringBuilder sb1 = new StringBuilder();
                     for (int i = ; i < a1.Length;i++)
                     {
                         sb1.Append(a1[i]);
                     }
                     if(string.IsNullOrEmpty(SmallestStr) ||
                        string.Compare(SmallestStr, sb1.ToString()) >
                       )
                     {
                         SmallestStr = sb1.ToString();
                     }
                 }
                 ST.Pop();
             }
         }
 
         public string SmallestFromLeaf(TreeNode root)
         {
             SearchTree(root);
             return SmallestStr;
         }
     }