191. Maximum Product Subarray

分正负数两种情况, 计算最大,最小乘积.
init: f_max[0] = f_min[0] = A[0]

class Solution:
    def maxProduct(self, A):
        # init
        pre_max = pre_min = 1
        ans = -float("inf")

        for v in A:
            if v < 0:
                max_val = max(v, pre_min * v)
                min_val = min(v, pre_max * v)
            else:
                max_val = max(v, pre_max * v)
                min_val = min(v, pre_min * v)
            ans = max(ans, max_val)
            pre_max = max_val
            pre_min = min_val
        return ans