238. Product of Array Except Self

class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        if (nums.size() == 0)   return vector<int>();
        vector<int> res(nums.size(), 1);
        for (int i = 1; i < nums.size(); i++) {
            res[i] = res[i-1] * nums[i-1];
        }
        int p = 1;
        for (int i = nums.size()-2; i >= 0; i--) {
            p = p * nums[i+1];
            res[i] = res[i] * p;
        }
        return res;
    }
};