class Solution {
    public int trap(int[] height) {
        int n = height.length;
        int[] l = new int[n];
        int[] r = new int[n];

        l[0] = height[0];
        for(int i = 1; i < n; i ++ ) l[i] = Math.max(l[i - 1], height[i]);

        r[n - 1] = height[n - 1];
        for(int i = n - 2; i >= 0; i -- ) r[i] = Math.max(r[i + 1], height[i]);

        int res = 0;
        for(int i = 1; i < n - 1; i ++ ) {
            res += Math.min(l[i], r[i]) - height[i];
        }

        return res;
    }
}