#include <algorithm>
#include <vector>

class Solution {
  public:
    int trap(const std::vector<int> &height) {
        int caught = 0;

        auto i = 0u;
        auto j = height.size() - 1;

        int max_l = height[i], max_r = height[j];
        while (i < j) {
            max_l = std::max(max_l, height[i]);
            max_r = std::max(max_r, height[j]);

            if (max_l < max_r) {
                caught += max_l - height[i++];
            } else {
                caught += max_r - height[j--];
            }
        }

        return caught;
    }
};