1 //两次dijkstra
2 #include <iostream>
3 #include <algorithm>
4
5 #define Faster ios::sync_with_stdio(false),cin.tie(0)
6 #define Read freopen("in.txt","r",stdin),freopen("out.txt","w",stdout)
7 #define Close fclose(stdin),fclose(stdout)
8 const int INF = 0xffffff;
9 const int maxn = 1005;
10 using namespace std;
11
12 int mp[maxn][maxn];
13 bool v[maxn];
14 bool v2[maxn];
15 int dis[maxn];
16 int dis2[maxn];
17
18 int n;
19
20 void dijkstra(int x){
21 for(int i = 1;i <= n;i++){
22 dis[i] = mp[x][i];
23 dis2[i] = mp[i][x];
24 v[i] = true;
25 v2[i] = true;
26 }
27 dis[x] = dis2[x] = 0;
28 v[x] = v2[x] = false;
29
30 for(int i = 1;i < n;i++){
31 int Min = INF;
32 int p;
33 for(int j = 1;j <= n;j++){
34 if(v[j] && dis[j] < Min){
35 Min = dis[j];
36 p = j;
37 }
38 }
39 if(Min == INF)
40 break;
41 v[p] = false;
42 for(int j = 1;j <= n;j++){
43 if(v[j] && dis[j] > dis[p] + mp[p][j])
44 dis[j] = dis[p] + mp[p][j];
45 }
46 }
47
48 for(int i = 0;i < n;i++){
49 int Min = INF;
50 int p;
51 for(int j = 1;j <= n;j++){
52 if(v2[j] && dis2[j] < Min){
53 Min = dis2[j] ;
54 p = j;
55 }
56 }
57 if(Min == INF)
58 break;
59 v2[p] = false;
60 for(int j = 1;j <= n;j++){
61 if(v2[j] && dis2[j] > dis2[p] + mp[j][p])
62 dis2[j] = dis2[p] + mp[j][p];
63 }
64 }
65 }
66
67 int main(){
68 Faster;
69 int m, k;
70 cin >> n >> m >> k;
71 for(int i = 0;i <= n;i++)
72 for(int j = 0;j <= n;j++){
73 mp[i][j] = INF;
74 if(i == j)
75 mp[i][j] = 0;
76 }
77 for(int i = 0;i < m;i++){
78 int x, y, z;
79 cin >> x >> y >> z;
80 if(mp[x][y] > z)
81 mp[x][y] = z;
82 }
83
84 dijkstra(k);
85
86 int ans = 0;
87 for(int i = 1;i <= n;i++){
88 ans = max(ans, dis[i]+dis2[i]);
89 //cout << "go: " << dis2[i] << " , ovel : " << dis[i] << endl;
90 }
91 cout << ans << endl;
92 return 0;
93 }
