P1807 最长路_NOI导刊2010提高（07） 

 1 //思路：把最长路转变为最短路，也就是将边权取负，最后再把负取正
 2 #include<bits/stdc++.h>
 3 using namespace std;
 4 const int maxm = 50000;
 5 const int maxn = 2000;
 6 struct enkidu {
 7     int y, nex, v;
 8 }e[maxm];
 9 int inf;
10 int lin[maxn], len = 0;
11 int n, m;
12 int dis[maxn];
13 bool vis[maxn], flag = 0;
14 
15 inline int read() {
16     int x = 0, y = 1;
17     char ch = getchar();
18     while(!isdigit(ch)) {
19         if(ch == '-') y = -1;
20         ch = getchar();
21     }
22     while(isdigit(ch)) {
23         x = (x << 1) + (x << 3) + ch - '0';
24         ch = getchar();
25     }
26     return x * y;
27 }
28 
29 inline void insert(int xx, int yy, int vv) {
30     e[++len].v = vv;
31     e[len].y = yy;
32     e[len].nex = lin[xx];
33     lin[xx] = len;
34 }
35 
36 queue<int> q;
37 inline void spfa(int st) {
38     q.push(st);
39     vis[st] = 1, dis[1] = 0;
40     while(!q.empty()) {
41         int k = q.front(); q.pop();
42         vis[k] = 0;
43         for(int i = lin[k]; i; i = e[i].nex) {
44             int to = e[i].y, val = e[i].v;
45             if(dis[k] + val < dis[to]) {
46                 dis[to] = dis[k] + val;
47                 if(!vis[to]) {
48                     vis[to] = 1;
49                     q.push(to);
50                 }
51             }
52             if(to == n) flag = 1;
53         }
54     }
55 }
56 
57 int main() {
58     memset(vis, 0, sizeof(vis));
59     memset(dis, 0x3f3f, sizeof(dis));
60     n = read(), m = read();
61     for(int i = 1; i <= m; ++i) {
62         int x, y, v;
63         x = read(), y = read(), v = read();
64         insert(x, y, -v);
65     }
66     spfa(1);
67     if(!flag) cout << -1 << '\n';
68     else cout << dis[n] * -1 << '\n'; 
69     return 0;
70 }

 

posted @ 2018-08-04 11:32  YuWenjue  阅读(143)  评论(0)    收藏  举报