Construct Binary Tree from String

此题要写出简洁的形式实在太难...因为题目确保了此树是一颗complete tree，所以对于结点的处理没有序列化二叉树那道题那么复杂了，思路其实都很直接，注意多位数和负数即可

class Solution(object):
    def str2tree(self, s):
        """
        :type s: str
        :rtype: TreeNode
        """
        if not s:
            return None
        stack = []
        n = len(s)
        cur = 0

        while cur < n:
            if s[cur].isdigit():
                num = 0
                while cur < n and s[cur].isdigit():
                    num = 10 * num + ord(s[cur]) - ord('0')
                    cur += 1
                stack.append(TreeNode(num))
            elif s[cur] == '-':
                num = 0
                cur += 1
                while cur < n and s[cur].isdigit():
                    num = 10 * num + ord(s[cur]) - ord('0')
                    cur += 1
                stack.append(TreeNode(-num))
            elif s[cur] == '(':
                stack.append(s[cur])
                cur += 1
            else:
                child = stack.pop()
                stack.pop()
                if stack[-1].left:
                    stack[-1].right = child
                else:
                    stack[-1].left = child
                cur += 1

        return stack[0]