poj1087 A Plug for UNIX & poj1459 Power Network (最大流)

读题比做题难系列……

poj1087

输入n，代表插座个数，接下来分别输入n个插座，字母表示。把插座看做最大流源点，连接到一个点做最大源点，流量为1。

输入m，代表电器个数，接下来分别输入m个电器，字符串表示。把电器看做最大流终点，连接到一个点做最大汇点，流量为1。

输入k，代表转换器个数，接下来分别输入k个转换器，每个插座输入两个字母a,b表示a可以连在b上。把转换器看做流，b->a，因为转换器无限提供，流量为无限大

代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <map>
#include <algorithm>
#include <string>

using namespace std;

const int N = 505;
const int INF = 0x7fffffff;

map<string, int>mymap;

int cap[N][N];
string str[N];
int flow[N];
int pre[N];

queue<int> q;

int BFS(int src, int des)
{
    int i;
    while (!q.empty()) q.pop();
    for (i = 1; i <= des; ++i) pre[i] = -1;
    pre[src] = 0;
    q.push(src);
    flow[src] = INF;
    while (!q.empty()) {
        int index = q.front();
        q.pop();
        if (index == des) break;
        for (i = 1; i <= des; ++i) {
            if (pre[i] == -1 && cap[index][i] > 0) {
                pre[i] = index;
                flow[i] = min(flow[index], cap[index][i]);
                q.push(i);
            }
        }
    }
    if (pre[des] == -1) return -1;
    return flow[des];
}

int maxFlow(int src, int des)
{
    int ans = 0;
    int in = 0;
    while ((in = BFS(src, des)) != -1) {
        int k = des;
        while (k != src) {
            int last = pre[k];
            cap[last][k] -= in;
            cap[k][last] += in;
            k = last;
        }
        ans += in;
    }
    return ans;
}

int main()
{
    int n, m, k, i;
    int cnt = 1;
    string str1,str2;
    cin >> n;
    for (i = 1; i <= n; ++i) {
        cin >> str1;
        mymap[str1] = ++cnt;
        cap[1][cnt] = 1;
    }
    cin >> m;
    for (i = 1; i <= m; ++i) {
        cin >> str[i] >> str2;
        if (!mymap[str[i]]) mymap[str[i]] = ++cnt;
        if (!mymap[str2]) mymap[str2] = ++cnt;
        cap[mymap[str2]][mymap[str[i]]] = 1;

    }
    cin >> k;
    for (i = 1; i <= k; ++i) {
        cin >> str1 >> str2;
        if (!mymap[str1]) mymap[str1] = ++cnt;
        if (!mymap[str2]) mymap[str2] = ++cnt;
        cap[mymap[str2]][mymap[str1]] = INF;
    }
    ++cnt;
    for (i = 1; i <= m; ++i) {
        cap[mymap[str[i]]][cnt] = 1;
    }
    cout << m - maxFlow(1, cnt);
    return 0;
}

　　

poj 1459

配个良心的地址，读起题来容易些：http://www.2cto.com/kf/201502/377406.html

方法和上题相同，最大源点连到发电站，流量就是发电站的最大电量，用户同理。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 105;
const int INF = 0x7fffffff;

int n, np, nc, m;

int flow[N];
int cap[N][N];
int pre[N];

queue<int>q;

int BFS(int src, int des)
{
        while (!q.empty()) {
                q.pop();
        }
        for (int i = 0; i <= des; ++i) {
                pre[i] = -1;
        }
        flow[src] = INF;
        pre[src] = 0;
        q.push(src);
        while (!q.empty()) {
                int index = q.front();
                q.pop();
                if (index == des) {
                        break;
                }
                for (int i = 1; i <= des; ++i) {
                        if (pre[i] == -1 && cap[index][i] > 0) {
                                pre[i] = index;
                                flow[i] = min(cap[index][i], flow[index]);
                                q.push(i);  //<---经常忘记这句！
                        }
                }
        }
        if (pre[des] == -1) {
                return -1;
        }
        return flow[des];
}

int maxFlow(int src, int des)
{
        int ans = 0;
        int in = 0;
        while ((in = BFS(src, des)) != -1) {
                int k = des;
                while (k != src) {
                        int last = pre[k];
                        cap[last][k] -= in;
                        cap[k][last] += in;
                        k = last;
                }
                ans += in;
        }
        return ans;
}

int main()
{
        int i;
        while (scanf("%d%d%d%d", &n, &np, &nc, &m) != EOF) {
                memset(cap, 0, sizeof cap);
                memset(flow, 0, sizeof flow);
                int u, v, z;

                for (i = 0; i < m; ++i) {
                        scanf(" (%d,%d)%d", &u, &v, &z);
                        cap[u + 1][v + 1] += z;
                }
                for (i = 0; i < np; ++i) { // power station
                        scanf(" (%d)%d", &u, &z);
                        cap[0][u + 1] = z;
                }
                for (i = 0; i < nc; ++i) { // consumer
                        scanf(" (%d)%d", &u, &z);
                        cap[u + 1][n + 1] = z;
                }
                printf("%d\n", maxFlow(0, n + 1));
        }
        return 0;
}

不明白为什么scanf的格式匹配会死循环，大概是回车的问题= =，加个空格就好了。