1018. Public Bike Management (30) （SPFA + DFS）

1018. Public Bike Management (30)

时间限制

400 ms

内存限制

65536 kB

代码长度限制

16000 B

判题程序

Standard

作者

CHEN, Yue

There is a public bike service in Hangzhou City which provides great convenience to the tourists from all over the world. One may rent a bike at any station and return it to any other stations in the city.

The Public Bike Management Center (PBMC) keeps monitoring the real-time capacity of all the stations. A station is said to be in perfect condition if it is exactly half-full. If a station is full or empty, PBMC will collect or send bikes to adjust the condition of that station to perfect. And more, all the stations on the way will be adjusted as well.

When a problem station is reported, PBMC will always choose the shortest path to reach that station. If there are more than one shortest path, the one that requires the least number of bikes sent from PBMC will be chosen.

Figure 1

Figure 1 illustrates an example. The stations are represented by vertices and the roads correspond to the edges. The number on an edge is the time taken to reach one end station from another. The number written inside a vertex S is the current number of bikes stored at S. Given that the maximum capacity of each station is 10. To solve the problem at S₃, we have 2 different shortest paths:

1. PBMC -> S₁ -> S₃. In this case, 4 bikes must be sent from PBMC, because we can collect 1 bike from S₁ and then take 5 bikes to S₃, so that both stations will be in perfect conditions.

2. PBMC -> S₂ -> S₃. This path requires the same time as path 1, but only 3 bikes sent from PBMC and hence is the one that will be chosen.

Input Specification:

Each input file contains one test case. For each case, the first line contains 4 numbers: C_max (<= 100), always an even number, is the maximum capacity of each station; N (<= 500), the total number of stations; S_p, the index of the problem station (the stations are numbered from 1 to N, and PBMC is represented by the vertex 0); and M, the number of roads. The second line contains N non-negative numbers C_i(i=1,...N) where each C_i is the current number of bikes at S_i respectively. Then M lines follow, each contains 3 numbers: S_i, S_j, and T_ij which describe the time T_ij taken to move betwen stations S_i and S_j. All the numbers in a line are separated by a space.

Output Specification:

For each test case, print your results in one line. First output the number of bikes that PBMC must send. Then after one space, output the path in the format: 0->S₁->...->S_p. Finally after another space, output the number of bikes that we must take back to PBMC after the condition of S_pis adjusted to perfect.

Note that if such a path is not unique, output the one that requires minimum number of bikes that we must take back to PBMC. The judge's data guarantee that such a path is unique.

Sample Input:

10 3 3 56 7 00 1 10 2 10 3 31 3 12 3 1

Sample Output:

3 0->2->3 0

题意： 给定n个点，m条边，然后从0这个点出发，达到指定的点，问哪条路最短？如果有长度相同的路径， 选择带自行车最少的那条， 如果还有相同的， 选择带回来自行车最少的那一条路

题解：首先用SPFA求出原点到每个点的最短路， 然后再做一遍DFS就可以了

这道题很容易弄错题意， 比如说我给出这一组数据

10 3 3 3

4 4 10

0 1 1

1 2 1

2 3 1

那么这个答案应该是 2 0->1->2->3 5

而不是 3 0->1->2->3 0

#include <cstdio>#include <cstring>#include <vector>#include <queue>using namespace std;const int INF = 0x3f3f3f3f;const int N = 510;int maze[N][N];int dis[N];int cap[N];int full, n, dest;void SPFA() {    bool vis[N];    memset(vis, false, sizeof(vis));    queue<int> q;    q.push(0);    vis[0] = true;    dis[0] = 0;    while (!q.empty()) {        int u = q.front();        q.pop();        for (int i = 0; i <= n; ++i) {            if (dis[u] + maze[u][i] < dis[i]) {                dis[i] = dis[u] + maze[u][i];                if (!vis[i]) {                    vis[i] = true;                    q.push(i);                }            }        }        vis[u] = false;    }}vector<int> path;int node[N];int total_cost = INF;int come_back = INF;void dfs(int cur, int cur_node, int from, int to) {    if (cur_node == dest) {        if (from < total_cost || (from == total_cost && to < come_back)) {            total_cost = from;            come_back = to;        }        else            return ;        path.clear();        for (int i = 0; i < cur; ++i)            path.push_back(node[i]);        return ;    }    for (int i = 0; i <= n; ++i) {        if (dis[cur_node] + maze[cur_node][i] == dis[i]) {            node[cur] = i;            if (cap[i] < full / 2) {                if (to > full / 2 - cap[i])                    dfs(cur + 1, i, from, to - (full / 2 - cap[i]));                else                    dfs(cur + 1, i, from + full / 2 - cap[i] - to, 0);            }            else                dfs(cur + 1, i, from, to + cap[i] - full / 2);        }    }}int main() {    int m;    scanf("%d%d%d%d", &full, &n, &dest, &m);    for (int i = 1; i <= n; ++i)        scanf("%d", cap + i);    memset(dis, INF, sizeof(dis));    memset(maze, INF, sizeof(maze));    for (int i = 0; i < m; ++i) {        int u, v, val;        scanf("%d%d%d", &u, &v, &val);        if (maze[u][v] > val)            maze[u][v] = maze[v][u] = val;    }    SPFA();    dfs(0, 0, 0, 0);    printf("%d 0", total_cost);    for (int i = 0; i < path.size(); ++i)        printf("->%d", path[i]);    printf(" %d\n", come_back);    return 0;}