1+ #include < bits/stdc++.h>
2+ 3+ using namespace std ;
4+ 5+ typedef long int li;
6+ typedef long long int lli;
7+ typedef vector<long long int > vlli;
8+ typedef vector<int > vi;
9+ typedef stack<long long int > slli;
10+ typedef pair<lli, lli> plli;
11+ 12+ class Graph
13+ {
14+ 15+ vector<vector<plli>> adjFor, adjRev;
16+ vector<tuple<lli, lli, lli>> edges;
17+ lli nodes;
18+ 19+ public:
20+ Graph (lli nodes)
21+ {
22+ this ->
adjFor .
resize (nodes, vector<plli>{});
23+ this ->
adjRev .
resize (nodes, vector<plli>{});
24+ this ->nodes = nodes;
25+ }
26+ 27+ void addDirectedEdge (lli src, lli dst, lli weight)
28+ {
29+ 30+ if (src == dst)
31+ {
32+ return ;
33+ }
34+ 35+ this ->adjFor [src].emplace_back (make_pair (dst, weight));
36+ this ->adjRev [dst].emplace_back (make_pair (src, weight));
37+ this ->edges .emplace_back (make_tuple (src, dst, weight));
38+ }
39+ 40+ void dijkstra (vector<vector<plli>> &adj, vector<lli> &distance, lli src)
41+ {
42+ priority_queue<plli, vector<plli>, greater<plli>> pq;
43+ distance[src] = 0 ;
44+ pq.push (make_pair (0 , src));
45+ while (!pq.empty ())
46+ {
47+ auto top = pq.top ();
48+ pq.pop ();
49+ if (top.first > distance[top.second ])
50+ continue ;
51+ for (auto v : adj[top.second ])
52+ {
53+ if (distance[v.first ] > top.first + v.second )
54+ {
55+ distance[v.first ] = top.first + v.second ;
56+ pq.push (make_pair (distance[v.first ], v.first ));
57+ }
58+ }
59+ }
60+ }
61+ 62+ void solve ()
63+ {
64+ 65+ vector<lli> distanceFor (this ->nodes , LONG_LONG_MAX);
66+ vector<lli> distanceRev (this ->nodes , LONG_LONG_MAX);
67+ 68+ this ->dijkstra (this ->adjFor , distanceFor, 0 );
69+ this ->dijkstra (this ->adjRev , distanceRev, this ->nodes - 1 );
70+ 71+ lli result = LONG_LONG_MAX;
72+ lli src, dst, weight;
73+ 74+ for (auto edge : this ->edges )
75+ {
76+ tie (src, dst, weight) = edge;
77+ if (distanceRev[dst] == LONG_LONG_MAX || distanceFor[src] == LONG_LONG_MAX)
78+ {
79+ continue ;
80+ }
81+ result = min (result, distanceFor[src] + distanceRev[dst] + (lli)floor (weight / 2 ));
82+ }
83+ cout << result << ' \n '
84+ }
85+ };
86+ 87+ void task ()
88+ {
89+ 90+ lli cities, flights;
91+ cin >> cities >> flights;
92+ 93+ lli src, dst, weight;
94+ 95+ Graph g (cities);
96+ 97+ for (lli i = 0 ; i < flights; i++)
98+ {
99+ cin >> src >> dst >> weight;
100+ g.addDirectedEdge (src - 1 , dst - 1 , weight);
101+ }
102+ 103+ g.solve ();
104+ }
105+ 106+ int main ()
107+ {
108+ 109+ ios::sync_with_stdio (0 );
110+ cin.tie (0 );
111+ 112+ task ();
113+ 114+ return 0 ;
115+ }
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