Construct Binary Tree from String — Python

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

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution(object):
    def str2tree(self, s):
        """
        :type s: str
        :rtype: TreeNode
        """
        def str2treeHelper(s, i):
            start = i
            if s[i] == '-': i += 1
            while i < len(s) and s[i].isdigit(): i += 1
            node = TreeNode(int(s[start:i]))
            if i < len(s) and s[i] == '(':
                i += 1
                node.left, i = str2treeHelper(s, i)
                i += 1
            if i < len(s) and s[i] == '(':
                i += 1
                node.right, i = str2treeHelper(s, i)
                i += 1
            return node, i

        return str2treeHelper(s, 0)[0] if s else None