Build Binary Expression Tree From Infix Expression — Python

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

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


class Solution(object):
    def expTree(self, s):
        """
        :type s: str
        :rtype: Node
        """
        def compute(operands, operators):
            right, left = operands.pop(), operands.pop()
            operands.append(Node(val=operators.pop(), left=left, right=right))

        precedence = {'+':0, '-':0, '*':1, '/':1}
        operands, operators, operand = [], [], 0
        for i in xrange(len(s)):
            if s[i].isdigit():
                operand = operand*10 + int(s[i])
                if i == len(s)-1 or not s[i+1].isdigit():
                    operands.append(Node(val=str(operand)))
                    operand = 0
            elif s[i] == '(':
                operators.append(s[i])
            elif s[i] == ')':
                while operators[-1] != '(':
                    compute(operands, operators)
                operators.pop()
            elif s[i] in precedence:
                while operators and operators[-1] in precedence and \
                      precedence[operators[-1]] >= precedence[s[i]]:
                    compute(operands, operators)
                operators.append(s[i])
        while operators:
            compute(operands, operators)
        return operands[-1]