P1807 最长路_NOI导刊2010提高（07）

//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
#include<bits/stdc++.h>
using namespace std;
const int maxm = 50000;
const int maxn = 2000;
struct enkidu {
    int y, nex, v;
}e[maxm];
int inf;
int lin[maxn], len = 0;
int n, m;
int dis[maxn];
bool vis[maxn], flag = 0;

inline int read() {
    int x = 0, y = 1;
    char ch = getchar();
    while(!isdigit(ch)) {
        if(ch == '-') y = -1;
        ch = getchar();
    }
    while(isdigit(ch)) {
        x = (x << 1) + (x << 3) + ch - '0';
        ch = getchar();
    }
    return x * y;
}

inline void insert(int xx, int yy, int vv) {
    e[++len].v = vv;
    e[len].y = yy;
    e[len].nex = lin[xx];
    lin[xx] = len;
}

queue<int> q;
inline void spfa(int st) {
    q.push(st);
    vis[st] = 1, dis[1] = 0;
    while(!q.empty()) {
        int k = q.front(); q.pop();
        vis[k] = 0;
        for(int i = lin[k]; i; i = e[i].nex) {
            int to = e[i].y, val = e[i].v;
            if(dis[k] + val < dis[to]) {
                dis[to] = dis[k] + val;
                if(!vis[to]) {
                    vis[to] = 1;
                    q.push(to);
                }
            }
            if(to == n) flag = 1;
        }
    }
}

int main() {
    memset(vis, 0, sizeof(vis));
    memset(dis, 0x3f3f, sizeof(dis));
    n = read(), m = read();
    for(int i = 1; i <= m; ++i) {
        int x, y, v;
        x = read(), y = read(), v = read();
        insert(x, y, -v);
    }
    spfa(1);
    if(!flag) cout << -1 << '
';
    else cout << dis[n] * -1 << '
'; 
    return 0;
}