TwitterRain

class Rain
{
    public static void main (String[] args)
    {
        int[] walls = {2, 5, 1, 2, 3, 4, 7, 7, 6};  
        System.out.println(calculateVolume(walls));
    }
    
    public static int calculateVolume(int[] walls) {
        
        int prev = 0;
        int max = 0;
        int localSum = 0;
        int totalSum = 0;
        for (int i=0; i<walls.length; i++) {
            if (i < prev && max==0) max = prev; 
                               
            if (walls[i] >= max) {
                max = 0;
                totalSum += localSum;
                localSum = 0;
            }
            
            if (max != 0) {
                localSum += max - walls[i];
            }
                       
            prev = walls[i];           
        }
        return totalSum;
    }
}

oh, shame ( bad solution, don't watch it

Not bad. The only disadvantage - it does not work for 99% cases