# LeetCode 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.

**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:**

* Could you solve it in `O(n)` time complexity and without using division?
* Could you solve it with `O(1)` constant space complexity? (The output array **does not** count as extra space for space complexity analysis.)

## Solution

[English Version in Youtube](https://youtu.be/_M6LXQqd33w)

[中文版解答Youtube Link](https://youtu.be/hdCuT7xxNAY)

[中文版解答Bilibili Link](https://www.bilibili.com/video/BV1rK4y1K72x/)

```
class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        vector<int>res(nums.size(), 1);
        
        for (int i = 1; i < nums.size(); i++) {
            res[i] = res[i-1] * nums[i-1];
        }
        
        int tmp = 1;
        for (int i = nums.size() - 1; i >= 0; i--) {
            res[i] *= tmp;
            tmp *= nums[i];
        }
        
        return res;
    }
};
```


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