Recover a Tree From Preorder Traversal — Python

# Time:  O(n)
# Space: O(h)

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


# iterative stack solution
class Solution(object):
    def recoverFromPreorder(self, S):
        """
        :type S: str
        :rtype: TreeNode
        """
        i = 0
        stack = []
        while i < len(S):
            level = 0
            while i < len(S) and S[i] == '-':
                level += 1
                i += 1
            while len(stack) > level:
                stack.pop()
            val = []
            while i < len(S) and S[i] != '-':
                val.append(S[i])
                i += 1
            node = TreeNode(int("".join(val)))
            if stack:
                if stack[-1].left is None:
                    stack[-1].left = node
                else:
                    stack[-1].right = node
            stack.append(node)
        return stack[0]


# Time:  O(n)
# Space: O(h)
# recursive solution
class Solution2(object):
    def recoverFromPreorder(self, S):
        """
        :type S: str
        :rtype: TreeNode
        """
        def recoverFromPreorderHelper(S, level, i):
            j = i[0]
            while j < len(S) and S[j] == '-':
                j += 1 
            if level != j - i[0]:
                return None
            i[0] = j
            while j < len(S) and S[j] != '-':
                j += 1
            node = TreeNode(int(S[i[0]:j]))
            i[0] = j
            node.left = recoverFromPreorderHelper(S, level+1, i)
            node.right = recoverFromPreorderHelper(S, level+1, i)
            return node

        return recoverFromPreorderHelper(S, 0, [0])