leetcode 238. Product of Array Except Self

238. Product of Array Except Self

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

Solve it without division and in O(n).

For example, given [1,2,3,4], return [24,12,8,6].

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

不能用除法，常数空间
思路是：两个数组，left[] right[] 分别存i元素左右的乘积。但是left[]用ret[]来存，right[]用变量来存。

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