解题报告

题目传送门

题意：

一个小镇，所有的街道都是单向的，这些街道都是从一个十字路口通往另一个十字路口，已知从任何十字路口出发，沿着这些街道行走，都不能回到同一个十字路口，也就是说不存在回路。

计算攻击这个小镇需要派的伞兵最少数目，这些伞兵要走遍小镇的所有十字路口，每个十字路口只由一个伞兵走到。每个伞兵在一个十字路口着陆，沿着街道可以走到其他十字路口。

思路：

用最小的伞兵覆盖街道，最小路径覆盖模型。把每个点拆成X1，Y1，这样建成二分图。最小路径覆盖=n-最大匹配数。

#include <iostream>
#include <cstring>
#include <cstdio>

using namespace std;
int k,n,m,mmap[1100][1100],vis[1100],pre[1100];
int dfs(int x)
{
    for(int i=1;i<=n;i++)
    {
        if(!vis[i]&&mmap[x][i])
        {
            vis[i]=1;
            if(pre[i]==-1||dfs(pre[i]))
            {
                pre[i]=x;
                return 1;
            }
        }
    }
    return 0;
}
int main()
{
    int i,j,u,v,a,t;
    scanf("%d",&t);
    while(t--)
    {
        memset(pre,-1,sizeof(pre));
        memset(mmap,0,sizeof(mmap));
        scanf("%d",&n);
        scanf("%d",&m);
        for(i=1;i<=m;i++)
        {
            scanf("%d%d",&u,&v);
            mmap[u][v]=1;
        }
        int ans=0;
        for(i=1;i<=n;i++)
        {
            memset(vis,0,sizeof(vis));
            ans+=dfs(i);
        }
        printf("%d\n",n-ans);
    }
}

Air Raid

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)

Problem Description

Consider a town where all the streets are one-way and each street leads from one intersection to another. It is also known that starting from an intersection and walking through town‘s streets you can never reach the same intersection i.e. the town‘s streets
form no cycles.

With these assumptions your task is to write a program that finds the minimum number of paratroopers that can descend on the town and visit all the intersections of this town in such a way that more than one paratrooper visits no intersection. Each paratrooper
lands at an intersection and can visit other intersections following the town streets. There are no restrictions about the starting intersection for each paratrooper.

Input

Your program should read sets of data. The first line of the input file contains the number of the data sets. Each data set specifies the structure of a town and has the format:

no_of_intersections

no_of_streets

S1 E1

S2 E2

......

Sno_of_streets Eno_of_streets

The first line of each data set contains a positive integer no_of_intersections (greater than 0 and less or equal to 120), which is the number of intersections in the town. The second line contains a positive integer no_of_streets, which is the number of streets
in the town. The next no_of_streets lines, one for each street in the town, are randomly ordered and represent the town‘s streets. The line corresponding to street k (k <= no_of_streets) consists of two positive integers, separated by one blank: Sk (1 <= Sk
<= no_of_intersections) - the number of the intersection that is the start of the street, and Ek (1 <= Ek <= no_of_intersections) - the number of the intersection that is the end of the street. Intersections are represented by integers from 1 to no_of_intersections.

There are no blank lines between consecutive sets of data. Input data are correct.

Output

The result of the program is on standard output. For each input data set the program prints on a single line, starting from the beginning of the line, one integer: the minimum number of paratroopers required to visit all the intersections in the town.

Sample Input

2
4
3
3 4
1 3
2 3
3
3
1 3
1 2
2 3

Sample Output

2
1

HDU1151_Air Raid(二分图/最小路径覆盖=n-最大匹配)