238. Product of Array Except Self

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

Problem

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]

Constraint: It's guaranteed that the product of the elements of any prefix or suffix of the array (including the whole array) fits in a 32 bit integer.

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) {
        vector<int> product;
        if (nums.empty()) return product;
        product.reserve(nums.size());
        product.push_back(1);
        for (int i = 1; i < nums.size(); ++i) product.push_back(nums[i - 1] * product[i - 1]);
        int suffix = 1;
        for (int i = nums.size() - 2; i >= 0; --i) {
            suffix *= nums[i + 1];
            product[i] *= suffix;
        }
        return product;
    }
};

• #prefix

• #suffix