[Leetcode]238. 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].
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.)
Example:
Input: [1,2,3,4] Output: [24,12,8,6]

An O(n^2) approach is easy to come up with, however, an O(n) is also not difficult with a small trick. We can do it with two-pass. On the first scanning, we store the accumulative products into an array; on the second scanning in the reversed order, we find the accumulative products along the scanning.

class Solution:
    def productExceptSelf(self, nums: List[int]) -> List[int]:
        ret = [1] * len(nums)
        for i in range(1,len(nums)):
            ret[i] = ret[i-1]*nums[i-1]
        right = 1
        for i in range(len(nums)-1,-1,-1):
            ret[i] *= right
            right *= nums[i]
        return ret
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