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.)

思路

从前向后累乘一遍，从后向前累乘一遍，一开始是用的两个数组，高分答案中遍历一遍即可。

代码

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

一遍过

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