29. Divide Two Integers

总的来说是减法没错，但是一个一个减去时间一定会超，所以做法是减去1倍，2倍， 4倍，没办法减以后再从1倍开始。

class Solution(object):
    def divide(self, dividend, divisor):
        """
        :type dividend: int
        :type divisor: int
        :rtype: int
        """
        positive = (dividend < 0) is (divisor < 0)
        dividend, divisor = abs(dividend), abs(divisor)
        res = 0
        while dividend >= divisor:
            temp, i = divisor, 1
            while dividend >= temp:
                dividend -= temp
                res += i
                i = i << 1
                temp = temp << 1
        if not positive:
            res = -res
        return min(max(-pow(2,31), res),pow(2,31)-1)