poj1135

最短路变形

#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;
}