classSolution{public:inttrap(vector<int>& height){
std::ios::sync_with_stdio(false);if(height.empty())return0;int n = height.size();
vector<int>left(n),right(n);
left[0]= height[0], right[n -1]= height[n -1];for(int i =1; i < n; i++)
left[i]=max(height[i], left[i -1]);for(int i = n -2; i >=0; i--)
right[i]=max(height[i], right[i +1]);int res =0;for(int i =0; i < n; i++)
res +=min(left[i], right[i])- height[i];return res;}};