class Solution {
public:
    int trap(vector<int>& height) {
        if (height.empty()) {
            return 0;
        }
        int n = height.size();

        vector<int> left(n);
        left[0] = height[0];
        for (int i = 1; i < n; ++i)
            left[i] = max(left[i - 1], height[i]);

        vector<int> right(n);
        right[n - 1] = height[n - 1];
        for (int i = n - 2; ~i; --i)
            right[i] = max(right[i + 1], height[i]);

        int res = 0;
        for (int i = 0; i < n; i ++ )
            res += min(left[i], right[i]) - height[i];
        return res;
    }
};