int calcWaterVolume(int count, int *x)
{
    int result = 0;
    int left, right, level;

    if (count < 3) { return 0; }

    for (left=0;  left<count-1;  ++left) {
        if (x[left+1] < x[left]) { break; }
    }
    for (right=count-1;  right>=1;  --right) {
        if (x[right-1] < x[right]) { break; }
    }
//    if (left == right) { return 0; }

    level = std::min(x[left], x[right]);

    for (int i=left; i<right; ++i) {
        if (level-x[i] > 0) { result += level-x[i]; }
    }

    return result;
}

    {
        //блоки для каждой координаты X
        std::vector<int> wall; 
        int volume=0;

        //тестовые значения
        wall.push_back(3);
        wall.push_back(1);
        wall.push_back(2);
        wall.push_back(5);

        if(wall.size())
        {
            //найдём максимальный и минимальный уровень
            int min_level=0;
            int max_level=0;
            {
                std::vector<int> mm=wall; 
                std::sort(mm.begin(),mm.end());
                min_level=*mm.begin();
                max_level=*mm.rbegin();
            }

            if(min_level>=1 && max_level>min_level)
            {
                //сканируем уровни, начиная снизу
                for(int level=min_level;level<=max_level;level++)
                {
                    //на текущем слева направо:
                    
                    //ищем первую стенку
                    int x1=0;
                    for(; x1!=wall.size(); x1++)
                    {
                        if(wall[x1]>=level)break;
                    }

                    //ищем последнюю стенку
                    int x2=wall.size()-1;
                    for(; x2>x1+1; x2--)
                    {
                        if(wall[x2]>=level)break;
                    }

                    //между стенами должен быть хотя бы 1 блок
                    if(x1+1<x2)
                    {
                        //сканируем пространство между x1 и x2 и считаем пустоту
                        for(int x=x1+1; x<x2-1; x++)
                        {
                            if(wall[x]<level)volume++;
                        }
                    }
                }
            }
        }

        //volume - результат
    }

#include <iostream>
#include <vector>

using namespace std;

typedef struct _data
{
    int level;
    int weight;
    _data(int h,int w) : level(h), weight(w) {}
}
data;

int calcVolume(int count, int *x)
{

    if (count<3) return 0;

    vector<data> v;
    int x_min = x[0];
    int current_level = x[0];
    int current_weight = 1;

    for(int i=1;i<count;i++)
    {
        if(x[i]<x_min) x_min = x[i];
        if(x[i]!=current_level)
        {
            v.push_back(data(current_level,current_weight));
            current_level = x[i];
            current_weight = 1;
        }
        else
        {
            current_weight++;
        }
     }
     v.push_back(data(current_level,current_weight));

    size_t size = v.size();
    for(size_t i=0; i<size; i++)
    {
        v[i].level-=x_min;
    }

    int volume = 0;
    vector<data> v_new;

    while(size>2)
    {
        for(size_t i=0; i<size; i++)
        {
            cout << v[i].level << "(" << v[i].weight << "), ";
        }
        cout << endl;

        current_level = v[0].level;
        current_weight = v[0].weight;
        for(size_t i=1; i<size; i++)
        {
            if(i != size-1)
            {
                if((v[i].level==0)&&(v[i-1].level>0)&&(v[i+1].level>0))
                {
                    volume += v[i].weight;
                    v[i].level += 1;
                }
            }
            if(v[i].level != current_level)
            {
                v_new.push_back(data(current_level-1,current_weight));
                current_level = v[i].level;
                current_weight = v[i].weight;
            }
            else
            {
                current_weight += v[i].weight;
            }
        }
        if(current_level>0)
        {
              v_new.push_back(data(current_level-1,current_weight));
        }
        v = v_new;
        size = v.size();
        v_new.clear();
    }
    return volume;
}

int main()
{
    int x[] = { 3, 1, 2, 1, 4, 1, 2, 1, 4 };
    cout << "Volume is " << calcVolume(sizeof(x)/sizeof(int),x) << endl;
    return 0;
}

#include <iostream>
#include <vector>

using namespace std;

typedef struct _data
{
    int level;
    int weight;
    _data(int h,int w) : level(h), weight(w) {}
}
data;

int calcVolume(int count, int *x)
{

    if (count<3) return 0;

    int start = count-1;
    for(int i=0;i<count-1; i++)
    {
        if(x[i]>x[i+1])
        {
            start = i;
            break;
        }
    }
    int stop = 0;
    for(int i=count-1;i>0; i--)
    {
        if(x[i]>x[i-1])
        {
            stop = i;
            break;
        }
    }

    vector<data> v;
    int x_min = x[start];
    int current_level = x[start];
    int current_weight = 1;

    for(int i=start;i<=stop;i++)
    {
        if(x[i]<x_min) x_min = x[i];
        if(x[i]!=current_level)
        {
            v.push_back(data(current_level,current_weight));
            current_level = x[i];
            current_weight = 1;
        }
        else
        {
            current_weight++;
        }
     }
     v.push_back(data(current_level,current_weight));

    size_t size = v.size();
    for(size_t i=0; i<size; i++)
    {
        v[i].level-=x_min;
    }

    int volume = 0;
    vector<data> v_new;

    while(size>2)
    {
        for(size_t i=0; i<size; i++)
        {
            cout << v[i].level << "(" << v[i].weight << "), ";
        }
        cout << endl;

        current_level = v[0].level;
        current_weight = v[0].weight;
        for(size_t i=1; i<size; i++)
        {
            if(i != size-1)
            {
                if((v[i].level==0)&&(v[i-1].level>0)&&(v[i+1].level>0))
                {
                    volume += v[i].weight;
                    v[i].level += 1;
                }
            }
            if(v[i].level != current_level)
            {
                if(current_level>0)
                {
                    v_new.push_back(data(current_level-1,current_weight));
                }
                current_level = v[i].level;
                current_weight = v[i].weight;
            }
            else
            {
                current_weight += v[i].weight;
            }
        }
        if(current_level>0)
        {
            v_new.push_back(data(current_level-1,current_weight));
        }
        v = v_new;
        size = v.size();
        v_new.clear();
    }
    return volume;
}

int main()
{
    int x[] = {   3, -1, -2, 8, -3, -1, 2, 0, -3  };
    cout << "Volume is " << calcVolume(sizeof(x)/sizeof(int),x) << endl;
    return 0;
}