poj 3268 Silver Cow Party

思路：单源最短路，套用spfa模板，这里需要构造逆向边，调用两次spfa。

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>#include <queue>using namespace std;const int N = 10009;const int inf = 1<<30;typedef struct node {    int v, w, next;}node;node p1[N], p2[N];bool vis[N];int head1[N], head2[N], dis1[N], dis2[N];queue <int> que;int n, m, x;void add_edge(int u, int v, int w, int i) {    p1[i].v = v;    p1[i].w = w;    p1[i].next = head1[u];    head1[u] = i;    p2[i].v = u;    p2[i].w = w;    p2[i].next = head2[v];    head2[v] = i;}void spfa(int dis[], node p[], int head[]) {    memset(vis, 0, sizeof(vis));    while (!que.empty())    que.pop();    int i, u, v;    for (i = 1; i <= n; ++i)        dis[i] = inf;    dis[x] = 0;    vis[x] = 1;    que.push(x);    while (!que.empty()) {        u = que.front();        que.pop();        for (i = head[u]; i != -1; i = p[i].next) {            v = p[i].v;            if (dis[v] > dis[u] + p[i].w) {                dis[v] = dis[u] + p[i].w;                if (!vis[v]) {                    vis[v] = true;                    que.push(v);                }            }        }        vis[u] = false;    }}int main(){    int i, u, v, w;    while (scanf("%d %d %d", &n, &m, &x) != EOF) {        memset(head1, -1, sizeof(head1));        memset(head2, -1, sizeof(head2));        for (i = 0; i < m; ++i) {            scanf("%d %d %d", &u, &v, &w);            add_edge(u, v, w, i);        }        int max_t = -1;        spfa(dis1, p1, head1);        spfa(dis2, p2, head2);        for (i = 1; i <= n; ++i) {            if (max_t < dis1[i] + dis2[i])                max_t = dis1[i] + dis2[i];        }        printf("%d\n", max_t);    }    return 0;}