Leetcode 238. Product of Array Except Self. Python (Prefix/Postfix Approach)

 238. Product of Array Except Self

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].

The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

You must write an algorithm that runs in O(n) time and without using the division operation.

 

Example 1:

Input: nums = [1,2,3,4]
Output: [24,12,8,6]

Example 2:

Input: nums = [-1,1,0,-3,3]
Output: [0,0,9,0,0]

 

Constraints:

• 2 <= nums.length <= 105
• -30 <= nums[i] <= 30
• The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

 

Follow up: Can you solve the problem in O(1) extra space complexity? (The output array does not count as extra space for space complexity analysis.)

class Solution:
    def productExceptSelf(self, n: List[int]) -> List[int]:
        ans = [1] * len(n)
        
        pre, pos = 1, 1
        
        for i in range(len(n)):
            ans[i] = pre
            pre *= n[i]
            
        for i in range(len(n)-1, -1, -1):
            ans[i] *= pos
            pos *= n[i]
            
        return ans
        

Explaination :