leetcode 306. Additive Number

class Solution(object):    def isAdditiveNumber(self, num):        """        :type num: str        :rtype: bool        """        n = len(num)        for i in range(1,n,1):            for j in range(i+1,n,1):                a,b = num[:i], num[i:j]                if a[0] == '0' and len(a) > 1:                    continue                if b[0] == '0' and len(b) > 1:                    continue                while j < n:                    c = str(int(a)+int(b))                    if num[j:j+len(c)] != c:                        break                    j += len(c)                    a,b = b,c                if j == n:                    return True        return False