536. Construct Binary Tree from String

# 因为是string，所以这道题主要是注意root.val可能是，负值，或是多位数
class Solution:
    def buildTree(self, s):
        if len(s) == 0:
            return None

        found = s.find('(')

        if found == -1:
            root = TreeNode(s)
            return root
        else:
            root = TreeNode(s[:found])

            start = found
            st = []
            for i in range(start, len(s)):
                if s[i] == '(':
                    st.append('(')
                elif s[i] == ')':
                    st.pop()

                if not st and start == found:
                    root.left = self.buildTree(s[start+1:i])
                    start = i + 1
                elif not st:
                    root.right = self.buildTree(s[start+1:i])

            return root

    def str2tree(self, s):
        """
        :type s: str
        :rtype: TreeNode
        """
        root = self.buildTree(s)
        return root