Encoded Tree Sums /homework in Swift

Given a tree encoded as a string, for every node, compute the sum of its subtree values and return the sums tree encoded as a string.

For example, the following tree is given as an encoded string "1{2{3,4},5{6}}" :

    1
   / \
  2   5
 / \   \
3   4   6

After computing the sums, the tree will look like the following:

    21
   /  \
  9    11
 / \     \
3   4     6

Therefore the output will be "21{9{3,4},11{6}}"

Example 1:

Input:  "1{2{3,4},5{6}}"

Output: "21{9{3,4},11{6}}"

Example 2:

Input:  "1{1,1{1,1,1},1}"

Output: "7{1,4{1,1,1},1}"