构建乘积数组

class Solution {
public:
    vector<int> multiply(const vector<int>& A) {
        vector<int> res;
        int n = A.size();
        res.push_back(1);
        //res中存每个累积的乘积
        for(int i = 1; i < n; i++){
            res.push_back(res.back() *A[i-1]);
        }
        //tmp累积从后往前的乘积
        int tmp = 1;
        for(int i = res.size()-1; i >= 0; i--){
            res[i] = res[i]*tmp;
            tmp *= A[i];
        }
        return res;
    }
};