Binary tree from string

link="https://www.lintcode.com/problem/880/"
"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""

class Solution:
    def str2tree(self, s):
        n,st,i=len(s),[],0
        while i<n:
            if s[i]=='-':
                i+=1
                num=0
                while i<n and s[i].isdigit():
                    num=num*10+int(s[i])
                    i+=1
                i-=1
                st.append(TreeNode(-num))
            elif s[i].isdigit():
                num=0
                while i<n and s[i].isdigit():
                    num=num*10+int(s[i])
                    i+=1
                i-=1
                st.append(TreeNode(num))
            elif s[i]==')':
                t=st.pop()
                if st[-1].left:
                    st[-1].right=t
                else:
                    st[-1].left=t
            i+=1
        return st[0]