LeetCode剑指 Offer 66. 构建乘积数组

//剑指 Offer 66. 构建乘积数组
//维护两个数组：l,r
//l[i]：原数组左边所有元素的乘积
//r[i]：原数组右边所有元素的乘积
//这样，ans[i]就等于l[i] * r[i]
class Solution {
public:
  vector<int> constructArr(vector<int>& a) {
    int n = a.size();
    vector<int>l(n, 1);
    vector<int>r(n, 1);
    for (int i = 1; i < n; i++) {
      l[i] = l[i - 1] * a[i - 1];
      r[n - 1 - i] = r[n - i] * a[n - i];
    }
    auto ans = a;
    for (int i = 0; i < n; i++)
      ans[i] = l[i] * r[i];
    return ans;
  }
};