[LeetCode 238] Product of Array Except Self

Problem

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 <= 10^5
`-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.)

Thoughts

We store preSum & postSum in result array

TypeScript

function productExceptSelf(nums: number[]): number[] {
    const res: number[] = [];
    
    let prefix: number = 1;
    for(let i = 0; i < nums.length; i++){
        if(i === 0){
            res[i] = 1;
        } else {
            prefix *= nums[i - 1]
            res[i] = prefix;
        }
    }
    
    let postfix: number = 1;
    for(let i = nums.length - 2; i >= 0; i--){
        postfix *= nums[i + 1]
        res[i] *= postfix;
    }
    
    return res;
};

Reference

https://leetcode.com/problems/product-of-array-except-self/

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