1 #include<iostream>
2 #include<algorithm>
3 #include<cstdio>
4 #include<cstring>
5 #include<queue>
6 using namespace std;
7 const int MAX_V = 1100;
8 const int MAX_E = 110000;
9 const int INF = 0x3f3f3f3f;
10
11 struct ENode
12 {
13 int to;
14 int w;
15 int Next;
16 };
17 ENode edegs[MAX_E];
18 int Head[MAX_V], tnt;
19 void Add_ENode (int a, int b, int w)
20 {
21 ++ tnt;
22 edegs[tnt].to= b;
23 edegs[tnt].w= w;
24 edegs[tnt].Next= Head[a];
25 Head[a]= tnt;
26 ++ tnt;
27 edegs[tnt].to= a;
28 edegs[tnt].w= w;
29 edegs[tnt].Next= Head[b];
30 Head[b]= tnt;
31 }
32
33 int Dis[MAX_V];
34 bool visit[MAX_V]; //是否已经遍历过;
35 int que[MAX_V]; // 模拟队列;
36 int outque[MAX_V]; //记录每个点出队列的次数，防止有负边，保证无负边的图可以省略;
37 bool SPFA(int s, int n)
38 {
39 /*s：起点； n：点的总数*/
40 int iq; //队列中元素个数;
41 memset(Dis, INF, sizeof(Dis));
42 memset(visit, false, sizeof(visit));
43 memset(outque, 0, sizeof(outque));
44 iq= 0;
45 que[iq ++]= s;
46 visit[s]= true;
47 Dis[s]= 0;
48 int i= 0;
49 while (i< iq)
50 {
51 int u= que[i];
52 visit[u]= false;
53 outque[u] ++;
54 if (outque[u]> n) return false; //当一个点进出队列超过n 次，必然有负边存在;
55
56 for (int k= Head[u]; k!= -1; k= edegs[k].Next)
57 {
58 /*bfs,和Dijkstra相似*/
59 int v= edegs[k].to;
60 if (Dis[v]> Dis[u]+ edegs[k].w)
61 {
62 Dis[v]= Dis[u]+ edegs[k].w;
63 if (! visit[v])
64 {
65 visit[v]= true;
66 que[iq ++]= v;
67 }
68 }
69 }
70 i ++;
71 }
72 return true;
73 }
74
75 void into()
76 {
77 /**初始化*/
78 memset(Head,-1,sizeof(Head));
79 tnt= -1;
80 }
81
82 int main()
83 {
84 int n, m;
85 int a, b, w;
86 while (~ scanf("%d %d", &n, &m))
87 {
88 into();
89 for (int i= 0; i< m; i ++)
90 {
91 scanf("%d %d %d", &a, &b, &w);
92 Add_ENode(a, b, w);
93 }
94 SPFA(1, n);
95 printf("%d\n", Dis[n]);
96 }
97 return 0;
98 }
