class Solution {
public:
    int trap(vector<int>& height) {
    if(height.empty()) {
        return 0;
    }
    stack<pair<int,int> > depth;
    int result = 0;
    auto ph = height[0];
    for(int i = 1; i < height.size(); ++i) {
        const auto h = height[i];
        if(h < ph) {
            depth.emplace(i - 1, ph - h);
        } else if (h > ph) {
            auto delta = h - ph;
            while(delta && !depth.empty()) {
                auto& d = depth.top();
                const auto actual = min(d.second, delta);
                d.second -= actual;
                delta -= actual;
                result += actual * (i - d.first - 1);
                if(d.second == 0) {
                    depth.pop();
                }
            }
        }
        ph = h;
    }
    return result;

    }
};