Find sum of all numbers that are formed from root to leaf path (code) expected time complexity O(n)
void DFSSearch(Node root, ref List<List<Int>> listOfRootToLeafPaths, ref List<int> curList)
{
 if (root != null)
  {
      if (curList == null) curList = new List<int>();
      curList.Add(root.value);
      If (root.left == null && root.right==null){
      listOfRootToLeafPaths.Add(curList);
      }
      DFSSearch(root.left, ref listOfRootToLeafPaths, ref curList);
      DFSSearch(root.right, ref listOfRootToLeafPaths, ref curList);
      curList.RemoveAt(curList.Length - 1); 
}
}

Combinations of  a string 
Void Combine (List<int> a, List<int> b, int start, int level, ref int Sum) 
{ 

   For (int I  = start ; I < a.Length; i++) 

   {  

       b[level] = a[i]; 

       Sum +=  b.ToNumber(); // digits multiplied by place order of tens

       If (I < a.Length) 

            Combine(a,b, start+1,level+1) 

       B.RemoveAt(B.Length - 1);

   } 

} 

Each combination can also have different permutations that can be added to Sum

Void Permute (List<int> a, List<int> b, bool[] used, ref Sum) 

{ 

  If ( b.Length == a.Length)  { Sum +=  b.ToNumber(); return;} 

  For (int I = 0; I < a.Length ; i++) 

  { 

    If (used[i]) continue; 

     used[i] = true; 

     B += A[i]; 

     Permute(a, b, used, ref Sum); 

     B.RemoveAt(B.Length - 1);

     used[i] = false; 

  } 

}