Product of Array Except Self

[shurintou]

Question

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

The question is not that difficult if there are no limit conditions, but since it need to be run in a O(n) time and and without using the division operation. I tried but didn't get a good answer.
And here is a good example I found. (It even has an O(1) Space Complexity!!)

/**
 * @param {number[]} nums
 * @return {number[]}
 */

var productExceptSelf = function (nums) {
    const resultArr = []
    for (let i = 0; i < nums.length; i++) {
        let accumulationProductVal = nums[i]
        if (resultArr.length !== 0)
            // This value is accumulated from start.
            accumulationProductVal = accumulationProductVal * resultArr[i - 1]
        resultArr.push(accumulationProductVal)
    }

    let accumulationProductVal = 1
    let i = nums.length - 1
    for (i; i > 0; i--) {
        resultArr[i] = resultArr[i - 1] * accumulationProductVal
        // This value is accumulated from end.
        accumulationProductVal = nums[i] * accumulationProductVal
    }
    resultArr[i] = accumulationProductVal

    return resultArr
}

You may not understand what this approach is doing just like I did first, but if you try to trace the operation you will understand it and find it is a great way to solve the problem.

The mechanism is very simple, which is that the each answer[i] is an product value accumulated start from the two side(the first and the last num) of the array, to the two num next to the num[i] itself.