poj The Settlers of Catan( 求图中的最长路 小数据量 暴力dfs搜索（递归回溯）)

The Settlers of Catan

Description

Within Settlers of Catan, the 1995 German game of the year, players attempt to dominate an island by building roads, settlements and cities across its uncharted wilderness.
You are employed by a software company that just has decided to
develop a computer version of this game, and you are chosen to implement
one of the game‘s special rules:

When the game ends, the player who built the longest road gains two extra victory points.

The problem here is that the players usually build complex road
networks and not just one linear path. Therefore, determining the
longest road is not trivial (although human players usually see it
immediately).

Compared to the original game, we will solve a simplified problem
here: You are given a set of nodes (cities) and a set of edges (road
segments) of length 1 connecting the nodes.

The longest road is defined as the longest path within the network
that doesn‘t use an edge twice. Nodes may be visited more than once,
though.

Example: The following network contains a road of length 12.

o      o--o      o
 \    /    \    /
  o--o      o--o
 /    \    /    o      o--o      o--o
           \    /
            o--o

Input

The input will contain one or more test cases.
The first line of each test case contains two integers: the number
of nodes n (2<=n<=25) and the number of edges m (1<=m<=25).
The next m lines describe the m edges. Each edge is given by the numbers
of the two nodes connected by it. Nodes are numbered from 0 to n-1.
Edges are undirected. Nodes have degrees of three or less. The network
is not neccessarily connected.

Input will be terminated by two values of 0 for n and m.

Output

For each test case, print the length of the longest road on a single line.

Sample Input

3 2
0 1
1 2
15 16
0 2
1 2
2 3
3 4
3 5
4 6
5 7
6 8
7 8
7 9
8 10
9 11
10 12
11 12
10 13
12 14
0 0

Sample Output

2
12
代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <algorithm>

using namespace std;

int map[30][30];
bool vis[30][30];
int n, m;
int MAX;

void dfs(int dd, int num )
{
	for(int i=0; i<n; i++)
	{
		if(map[dd][i]==1 && vis[dd][i]==0 )
		{
			vis[dd][i]=1; vis[i][dd]=1;
			dfs(i, num+1);
			vis[dd][i]=0; vis[i][dd]=0;
		}
	}
	if(num>MAX) MAX=num;
}

int main()
{
	int i, j;
	int u, v;
	while(scanf("%d %d", &n, &m)!=EOF)
	{
		if(n==0 && m==0 )break;
		memset(map, 0, sizeof(map ));

		for(i=0; i<m; i++)
		{
			scanf("%d %d", &u, &v);
			map[u][v]=1; map[v][u]=1;
		}
		MAX=-1;
        int cnt;
		for(i=0; i<n; i++)
		{
		    cnt=0;
			memset(vis, false, sizeof(vis));
			dfs(i, 0);
			//if(cnt>MAX) MAX=cnt;
		}
		printf("%d\n", MAX );
	}
	return 0;
}