acwing1128. 信使( spfa模板题)

题意

代码

#include <bits/stdc++.h>
using namespace std;
#define db  double
#define ll  long long
#define Pir pair<int, int>
#define fi  first
#define se  second
#define pb  push_back
#define m_p make_pair
#define inf 0x3f3f3f3f
#define INF 0x3f3f3f3f3f3f3f3f
/*==========ACMer===========*/
const int N = 105, M = 405;
int n, m;
struct Edge
{
    
    
    int v, w, ne;
} e[M];
int h[N], tot;

void add(int u, int v, int w)
{
    
    
    e[++ tot] = {
    
     v, w, h[u] }; h[u] = tot;
}

int dis[N], vis[N];
void spfa()
{
    
    
    memset(dis, inf, sizeof dis);
    dis[1] = 0;
    queue<int> q;
    q.push(1);
    while (q.size())
    {
    
    
        int u = q.front(); q.pop();
        vis[u] = 0;
        for (int i = h[u]; i; i = e[i].ne) {
    
    
            int v = e[i].v, w = e[i].w;
            if (dis[v] > dis[u] + w)
            {
    
    
                dis[v] = dis[u] + w;
                if (! vis[v])           //这个 if 不能和上个 if 放到一个 if 语句中去
                {
    
    
                    q.push(v);
                    vis[v] = 1;
                }
            }
        }
    }
}


int main()
{
    
    
    scanf("%d %d", &n, &m);
    int u, v, w;
    for (int i = 0; i < m; i ++) {
    
    
        scanf("%d %d %d", &u, &v, &w);
        add(u, v, w);
        add(v, u, w);
    }

    spfa();

    int mx = 0;
    for (int i = 1; i <= n; i ++)
        mx = max(mx, dis[i]);
    if (mx == inf) mx = -1;
    printf("%d\n", mx);


    return 0;
}