【luogu P1807 最长路_NOI导刊2010提高（07）】 题解

题目链接：https://www.luogu.org/problemnew/show/P1807
求最大路？就是把权值取相反数跑最短路。

#include <cstdio>
#include <iostream>
#include <cstring>
#include <queue>
#include <cmath>
#include <algorithm>
using namespace std;
const int maxn = 100000;
const int inf = 0x7fffffff;
bool vis[maxn];
struct edge{
    int from, next, len, to;
}e[maxn];
int head[maxn], cnt, dis[maxn], ans, m, n, u, v, w;
queue<int> q;
void add(int u, int v, int w)
{
    e[++cnt].from = u;
    e[cnt].to = v;
    e[cnt].len = w;
    head[u] = cnt;
}
int SPFA()
{
    while(!q.empty())
    {
        int now = q.front();
        q.pop();
        vis[now] = 0;
        for(int i = head[now]; i; i = e[i].next)
        {
            if(dis[e[i].to] > dis[now] + e[i].len)
            {
                dis[e[i].to] = dis[now] + e[i].len;
                if(!vis[e[i].to])
                {
                    vis[e[i].to] = 1;
                    q.push(e[i].to);
                }   
            }
        }
    }
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i = 1; i <= m; i++)
    {
        scanf("%d%d%d",&u,&v,&w);
        w = w*-1;
        add(u,v,w);
    }
    int s = 1;
    dis[s] = 0;
    q.push(s);
    vis[s] = 1;
    SPFA();
    if(dis[n] != 0)
    printf("%d",abs(dis[n]));
    else
    printf("-1");
    return 0;
}