leetcode+ array，线性时间，求累积，求出某一个位置前后的累积

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