poj1135

最短路变形

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#include <iostream>

#include <cstdlib>

#include <cstdio>

#include <cstring>

using namespace std;



#define maxn 505

#define maxm maxn * maxn

#define inf 0x3f3f3f3f



struct Edge

{

    int v, w, next;

}edge[maxm];



int n, m;

int head[maxn];

int edge_num;

int q[maxn];

int dist[maxn];

bool vis[maxn];



void addedge(int a, int b, int w)

{

    edge[edge_num].v = b;

    edge[edge_num].w = w;

    edge[edge_num].next = head[a];

    head[a] = edge_num++;

}



void input()

{

    memset(head, -1, sizeof(head));

    edge_num = 0;

    for (int i = 0; i < m; i++)

    {

        int a, b, w;

        scanf("%d%d%d", &a, &b, &w);

        a--;

        b--;

        addedge(a, b, w);

        addedge(b, a, w);

    }

}



bool relax(int u, Edge &e)

{

    if (dist[e.v] > dist[u] + e.w)

    {

        dist[e.v] = dist[u] + e.w;

        return true;

    }

    return false;

}



bool spfa(int x)

{

    memset(vis, 0, sizeof(vis));

    for (int i = 0; i < n; i++)

        dist[i] = inf;

    int front = 0;

    int rear = 1;

    q[front] = x;

    dist[x] = 0;

    vis[x] = true;

    while (front != rear)

    {

        int temp = q[front++];

        if (front == maxn)

            front = 0;

        vis[temp] = false;

        for (int i = head[temp]; i != -1; i = edge[i].next)

        {

            if (relax(temp, edge[i]) && !vis[edge[i].v])

            {

                q[rear++] = edge[i].v;

                if (rear == maxn)

                    rear = 0;

            }

        }

    }

    return false;

}



double get_time(int a, int b, int w)

{

    if (a > b)

        swap(a, b);

    if (a + w <= b)

        return b;

    return b + (w - (b - a)) * 1.0 / 2;

}



void work()

{

    int ans = 0;

    for (int i = 0; i < n; i++)

        if (dist[ans] < dist[i])

            ans = i;

    int a, b;

    double max_time;

    max_time = dist[ans];

    for (int u = 0; u < n; u++)

        for (int i = head[u]; ~i; i = edge[i].next)

        {

            int v = edge[i].v;

            double temp = get_time(dist[u], dist[v], edge[i].w);

            if (temp > max_time)

            {

                a = min(u, v);

                b = max(u, v);

                max_time = temp;

            }

        }

    if (max_time > dist[ans])

        printf("The last domino falls after %.1f seconds, between key dominoes %d and %d.\n", max_time, a + 1, b + 1);

    else

        printf("The last domino falls after %.1f seconds, at key domino %d.\n", (double)dist[ans], ans + 1);

}



int main()

{

//    freopen("t.txt", "r", stdin);

    int t = 0;

    while (scanf("%d%d", &n, &m), n | m)

    {

        printf("System #%d\n", ++t);

        input();

        spfa(0);

        work();

        puts("");

    }    

    return 0;

}