238. Product of Array Except Self

Problem Link

Description

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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 input is generated such that answer[i] 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.)

Solution

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Python3

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

C++

class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        int N = nums.size();
        vector<int> res(N, 1);
 
        int curr = 1;
        // construct prefix
        for (int i = 0; i < N; i++) {
            res[i] *= curr;
            curr *= nums[i];
        }
 
        curr = 1;
        // construct suffix
        for (int i = N - 1; i >= 0; i--) {
            res[i] *= curr;
            curr *= nums[i];
        }
 
        return res;
    }
};