Trapping Rain Water

#include <iostream>
#include <stack>
using namespace std;

class Solution {
public:
    int trap(int A[], int n) {

        if (n<3){
            return 0;
        }

        stack<int> water_stack;
        int begin=0;
        int res=0;

        for (int i =0;i<n;++i){
            if (A[i]>0 && water_stack.empty()){
                water_stack.push(A[i]);
                begin=A[i];
                continue;
            }
            if (A[i]<begin){
                water_stack.push(A[i]);
                continue;
            }
            if (A[i]>=begin && begin!=0){
                while(!water_stack.empty()){
                    //cout<<"A"<<i<<": "<<A[i];
                    //cout<<" begin: "<<begin;
                    //cout<<" top: "<<water_stack.top()<<endl;
                    res+=begin-water_stack.top();
                    water_stack.pop();
                }
                water_stack.push(A[i]);
                begin=A[i];
            }
            //cout<<"res: "<<res<<endl;
        }

        int end=0;
        while(!water_stack.empty()){
            if(water_stack.top()>=end){
                end=water_stack.top();
                water_stack.pop();
            }
            else{
                res+=end-water_stack.top();
                water_stack.pop();
            }
        }

        return res;
    }
};


int main(int argc, char** argv) {
    Solution s;
    int A[]={0,1,0,2,1,0,1,3,2,1,2,1};
    cout<<s.trap(A,12);
    return 0;
}