Product of Array Except Self

Description

Given an array nums of n integers where n > 1, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].

Example:

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

Note: Please solve it without division and in O(n).

Follow up: Could you solve it with constant space complexity? (The output array does not count as extra space for the purpose of space complexity analysis.)

Solution

class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        int n = nums.size();
        vector<int> ans(n);
        int prod = 1;
        for(int i = 0; i < n; ++i){
            ans[i] = prod;
            prod *= nums[i];
        }
        // at this point, ans[i] = product of first i elements
        prod = 1;
        for(int j = n - 1; j >= 0; --j){
            ans[j] *= prod;
            prod *= nums[j];
        }
        return ans;
    }
};