0238-Product-of-Array-Except-Self.py

'''
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.)
'''

# Using Postfix and Prefix Product

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

# Similar method with small changes

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