Lintcode Building Outline

class Solution {
public:
    /**
     * @param buildings: A list of lists of integers
     * @return: Find the outline of those buildings
     */
     
    set<int> loc;
    
    map<int,vector<int>> st,en;
    
    multiset<int> H;
    
    
    vector<vector<int>> buildingOutline(vector<vector<int>> &buildings) {
        // write your code here
        vector<vector<int>> ret;
        for (auto x : buildings)
        {
            loc.insert(x[0]);
            loc.insert(x[1]);
            st[x[0]].push_back(x[2]);
            en[x[1]].push_back(x[2]);
        }
        
        int H0 = 0;
        int X0 = 0;
        H.insert(0);
        for (auto X1 : loc)
        {
            auto H1 = *H.rbegin();
            if (H1)
            {
                if (ret.empty() || H1 != ret[ret.size() - 1][2])
                ret.push_back(vector<int>{X0,X1,H1});
                else ret[ret.size() - 1][1] = X1;
            }
            for (auto x : st[X1]) H.insert(x);
            for (auto x : en[X1]) H.erase(H.find(x));
            X0 = X1;
        }
        
        return ret;
    }
};