#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;}