https://leetcode.com/problems/product-of-array-except-self/
Given an array nums of n integers where n > 1,
return an array output such that output[i] is equal to
the product of all the elements of nums except nums[i].
Example:
Input: [1,2,3,4]
Output: [24,12,8,6]
Note: Please solve it without division and in O(n).
Follow up: Could you solve it with constant space complexity? (The output array does not count as extra space for the purpose of space complexity analysis.)