洛谷 P1807 最长路_NOI导刊2010提高（07）

最长路

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <vector>
using namespace std;
//Mystery_Sky
//
#define maxn 1000010
#define maxm 5000050
#define INF 0x3f3f3f3f
queue <int> q;
int ind[maxn], dis[maxn];
struct Edge{
    int to, next, w;
}edge[maxn];
int head[maxn], cnt, ans;
int n, m;
inline void add_edge(int u, int v, int w)
{
    edge[++cnt].to = v;
    edge[cnt].w = w;
    edge[cnt].next = head[u];
    head[u] = cnt;
}
inline void topo()
{
    for(int i = 1; i <= n; i++) {
        if(!ind[i]) {
            q.push(i); 
        }
    }
    dis[1] = INF;
    while(!q.empty()) {
        int u = q.front();
        q.pop();
        for(int i = head[u]; i ; i = edge[i].next) {
            int v = edge[i].to;
//          printf("%d %d %d\n", u, v, edge[i].w);
            ind[v]--;
            dis[v] = max(dis[v], dis[u] + edge[i].w);
            if(!ind[v]) q.push(v); 
        } 
    }
    
}

int u, v, w;
int main() {
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= m; i++) {
        scanf("%d%d%d", &u, &v, &w);
        add_edge(u, v, w);
        ind[v]++;
    }
    topo();
    if(dis[n]) printf("%d\n", dis[n]-INF);
    else printf("-1\n");
    return 0;
}