poj 3159 差分约束+spfa

差分约束第一题，想明白以后就可以建图了。由于此题数据特殊，队列优化的spfa会超时，可以改成用栈来优化。

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;

const int INF = 9999999;
const int N = 30001;
const int M = 150000;
int head[N];
int dist[N];
bool visit[N];
int n, m, e;

struct Edge
{
    int v, next, w;
} edge[M];

void addEdge( int u, int v, int w )
{
    edge[e].v = v;
    edge[e].w = w;
    edge[e].next = head[u];
    head[u] = e++;
}

void spfa( int s )
{
    for ( int i = 1; i <= n; i++ )
    {
        dist[i] = INF;
        visit[i] = false;
    }
    dist[s] = 0;
    visit[s] = true;
    int q[N], top = 0;
    q[top++] = s;
    while ( top )
    {
        int u = q[--top];
        visit[u] = false;
        for ( int j = head[u]; j != -1; j = edge[j].next )
        {
            int v = edge[j].v, w = edge[j].w;
            if ( dist[v] > dist[u] + w )
            {
                dist[v] = dist[u] + w;
                if ( !visit[v] )
                {
                    q[top++] = v;
                    visit[v] = true;
                }
            }
        }
    }
}

int main ()
{
    while ( scanf("%d%d", &n, &m) != EOF )
    {
        e = 0;
        memset( head, -1, sizeof(head) );
        while ( m-- )
        {
            int u, v, w;
            scanf("%d%d%d", &u, &v, &w);
            addEdge( u, v, w );
        }
        spfa(1);
        printf("%d
", dist[n]);
    }
    return 0;
}