Given an integer array nums, find the contiguous subarray within an array (containing at least one number) which has the largest product.
Example 1:
Input: [2,3,-2,4]
Output: 6
Explanation: [2,3] has the largest product 6.
Example 2:
Input: [-2,0,-1]
Output: 0
Explanation: The result cannot be 2, because [-2,-1] is not a subarray.
负负得正，正负得负，所以我们无法确定正负手，因此必须将最大的正数和最小的负数同时记录，
一旦遇到负数，便需要交换两者，在这个过程之中，记录产生的最大值，便是answer
