leetcode 282. 给表达式添加运算符

给定一个仅包含0-9的字符串和一个目标值，返回在数字之间添加了二元运算符(不是一元的) +、-或*之后所有能得到目标值的情况。

例如:

"123", 6 -> ["1+2+3", "1*2*3"]
"232", 8 -> ["2*3+2", "2+3*2"]
"105", 5 -> ["1*0+5","10-5"]
"00", 0 -> ["0+0", "0-0", "0*0"]
"3456237490", 9191 -> []

#coding=utf-8
import itertools
import re
 
class Solution(object):
    def addOperators(self, num, target):
        if num:
            count = len(num)-1
            rnum = num[-1]
 
            s = "['','+','-','*'],"
            ss = ""
            for c in range(count):
                ss += s
 
            ls = []
            for item in itertools.product(*eval(ss[0:-1])):
                zty = ''.join(list(itertools.chain.from_iterable(zip(list(num),item))))+rnum
                if eval(zty) != target or re.match(r".*([\+\-\*]0)\d.*",zty,re.M|re.I):
                    continue
                else:
                    ls.append(zty)
            return ls
 
        return []
 
print Solution().addOperators("", 5)