Feedback welcome!

```
public class Solution {
public int trap(int[] height) {
int[] fwd = new int[height.length];
int prev = 0;
for (int i = 0; i < fwd.length; i++) {
if (prev != 0 && height[i] < prev) {
fwd[i] = prev;
}
prev = Math.max(prev,height[i]);
}
prev = 0;
int sum = 0;
for (int i = height.length-1; i >= 0; i--) {
if (prev != 0 && fwd[i] != 0 && height[i] < Math.min(prev,fwd[i])) {
sum += Math.min(fwd[i],prev) - height[i];
}
prev = Math.max(prev,height[i]);
}
return sum;
}
}
```