Given a network of servers and the connections between them with their latency, find the fastest path for sending a message.
A straight-forward path finding problem. Dijkstra will do and Bellman Ford will get TLE.
#include <algorithm>
#include <iostream>
#include <limits>
#include <vector>
#include <queue>
#include <map>
using namespace std;
vector<map<int, int> > graph;
vector<int> dijkstraD, dijkstraP;
void dijkstra(int s) {
dijkstraD = vector<int>(graph.size(), numeric_limits<int>::max());
dijkstraP = vector<int>(graph.size(), -1);
priority_queue<pair<int, int> > q;
dijkstraP[s] = -s - 1;
dijkstraD[s] = 0;
q.push(make_pair(dijkstraD[s], s));
while (!q.empty()) {
pair<int, int> p = q.top();
q.pop();
if (dijkstraP[p.second] < 0) {
dijkstraP[p.second] = -dijkstraP[p.second] - 1;
for (map<int, int>::iterator itr = graph[p.second].begin();
itr != graph[p.second].end(); ++itr)
if (dijkstraP[itr->first] < 0
&& dijkstraD[p.second] + graph[p.second][itr->first] < dijkstraD[itr->first]) {
dijkstraP[itr->first] = -p.second - 1;
dijkstraD[itr->first] = dijkstraD[p.second] + graph[p.second][itr->first];
q.push(pair<int, int>(-dijkstraD[itr->first], itr->first));
}
}
}
}
int main() {
int N, n, m, s, t;
cin >> N;
for (int caseI = 0; caseI < N; ++caseI) {
// input
cin >> n >> m >> s >> t;
graph.clear();
for (int i = 0; i < n; ++i)
graph.push_back(map<int, int>());
for (int i = 0, a, b, w; i < m; ++i)
cin >> a >> b >> w, graph[a][b] = graph[b][a] = w;
// solve
dijkstra(s);
cout << "Case #" << (caseI + 1) << ": ";
if (dijkstraD[t] == numeric_limits<int>::max())
cout << "unreachable";
else
cout << dijkstraD[t];
cout << endl;
}
return 0;
}
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