SPOJ #440. The Turtle´s Shortest Path

Coding a Dijkstra is not hard. %70 of my time spent on tackling TLE, as my last post.

Dijkstra works in BFS manner, but at each step, it picks the shortest child greedily and then relax all other neighbors.

Data Structure: since there could be up to 10000 cities, storing distance for all pairs of cities are not allowed - 300+M space exceeds SPOJ's 256M limit. So I simply used std::map (my another version used GNU's hash_map but not tried its performance) as the adjacent linked list to store graph; and map<string, int> to store city name - index mapping. Again, hash_map could be better.

Anyway, here's my AC code, 11.34s:

//

#include <vector>
#include <cstdio>
#include <iostream>
#include <queue>
#include <map>
using namespace std;

/*
//////////////////////////////////
//    Components for GNU's hash_map
#include <backward/hash_map>
using namespace __gnu_cxx;
namespace __gnu_cxx
{
  template<>
  struct hash<std::string>
  {
    hash<char*> h;
    size_t operator()(const std::string &s) const
    {
      return h(s.c_str());
    };
  };
}
*/
//////////////////////////////////

const int MaxCity = ;
const int MaxDist = 0xFFFFFFF;
map<string, int> hmap;
typedef map<int, map<int, int> > DHash;
DHash distMap;

bool visited[MaxCity] = {false};
int dist[MaxCity] = {MaxDist};

/////////////////////////////////////
void add_dist(int srcInx, int toInx, int cost)
{
    distMap[srcInx][toInx] = cost;
}
int get_dist(int srcInx, int toInx)
{
    DHash::iterator iter = distMap.find(srcInx);
    if(iter != distMap.end())
    {
        map<int, int>::iterator iter2 = iter->second.find(toInx);
        if(iter2 != iter->second.end())
        {
            return iter2->second;
        }
    }
    return MaxDist;
}
/////////////////////////////////////
struct nComp
{
 int operator() (const pair<int,int>& p1, const pair<int,int> &p2)
 {
     return p1.second > p2.second;
 }
};
///////////////////////////////////////
void clearGraph()
{
    hmap.clear();
    distMap.clear();
}

void clearRuntime(int citycnt)
{
    for(int i = ; i < citycnt; i ++)
    {
        visited[i] = false;
        dist[i] = MaxDist;
    }
}
///////////////////////////////////////////
int find_shortest_path(int cinx1, int cinx2)
{
    int vcnt = hmap.size();

    priority_queue<pair<int, int>, vector<pair<int, int> >, nComp> q;
    //    Init
    for(int i = ; i < vcnt; i ++)
    {
        dist[i] = MaxDist;
        visited[i] = false;
    }
    dist[cinx1 - ] = ;
    q.push(make_pair(cinx1-, ));

    //    Go
    while(!q.empty())
    {
        pair<int,int> n = q.top(); q.pop();

        //    Greedy, find the unvisited min-cost node
        int minInx = n.first;
        int minCost = n.second;
        if(minCost == MaxDist)    break;
        visited[minInx] = true;
        if(minInx == cinx2 - ) return dist[cinx2 - ];

        //    Relax neighbors
        DHash::iterator iter = distMap.find(minInx);
        map<int,int> &child = iter->second;
        for(map<int,int>::iterator iMap = child.begin(); iMap != child.end(); iMap ++)
        {
            int i = iMap->first;
            int gDist = iMap->second;
            int alt = dist[minInx] + gDist;
            if(alt < dist[i])
            {
                dist[i] = alt;
                //cout << "\t\trelaxed to " << alt << " " << minInx << " to" << i <<endl;
                q.push(make_pair(i, dist[i]));
            }
        }
    }

    return MaxDist;
}
/////////////////////////
#define gc getchar
int read_int()
{
  char c = gc();
  while(c<'' || c>'') c = gc();
  int ret = ;
  while(c>='' && c<='') {
    ret =  * ret + c - ;
    c = gc();
  }
  return ret;
}
/////////////////////////
int main()
{
    int runcnt = read_int();
    //cout << "Run " << runcnt << endl;
    while(runcnt --)
    {
        clearGraph();
        int ccnt = read_int();
        for(int ic = ; ic <= ccnt; ic ++)
        {
            // cityName -> cityInx
            string city_name; cin >> city_name;
            int nnbr = read_int();
            hmap.insert(map<string, int>::value_type(city_name, ic));
            //cout << city_name << "->" << ic << endl;

            //    Fill out adjacency matrix
            while(nnbr--)
            {
                int cinx = read_int();
                int cost = read_int();
                add_dist(ic-, cinx-, cost);
                //cout << ic << " <-> " << cinx << " = " << cost << endl;
            }
        }

        int qcnt  = read_int();
        while(qcnt --)
        {
            string c1, c2;
            cin >> c1 >> c2;
            int cinx1 = hmap[c1];
            int cinx2 = hmap[c2];
        //    cout << c1 << "-" << c2 << " = " << cinx1 << " - " << cinx2 << endl;
            clearRuntime(ccnt);
            cout << find_shortest_path(cinx1, cinx2) << endl;
        }
        getchar();
        getchar(); // should return 0xA (enter)
    }

    return ;
}