题目大意：给你一个朋友之间的关系让你判断所有人之间的朋友“链”的最大长度，如果大于7就不可以，否则输出最大值。

枚举n个点进行n次spfa。

Friend Chains

3
XXX
YYY
ZZZ
2
XXX YYY
YYY ZZZ
0

2

#include <algorithm>
#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <iomanip>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>
#include <time.h>
#include <stack>
#include <map>
#include <set>
#define eps 1e-8
///#define LL long long
#define LL __int64
#define INF 0x3f3f3f
#define PI 3.1415926535898
#define mod 1000000007


using namespace std;

const int maxn = 1010;

map<string, int> mp;


string str;
string str1;


struct node
{
    int v;
    int w;
    int next;
}f[maxn*maxn/2];

int head[maxn*maxn/2];
int cnt;

void init()
{
    cnt = 0;
    memset(head, -1, sizeof(head));
}


void add(int u, int v, int w)
{
    f[cnt].v = v;
    f[cnt].w = w;
    f[cnt].next = head[u];
    head[u] = cnt++;

    f[cnt].v = u;
    f[cnt].w = w;
    f[cnt].next = head[v];
    head[v] = cnt++;

}

int spfa(int x, int n)
{
    bool vis[maxn];
    int dis[maxn];
    queue<int>que;
    for(int i = 1; i <= n; i++)
    {
        dis[i] = INF;
        vis[i] = false;
    }
    dis[x] = 0;
    vis[x] = true;
    que.push(x);
    while(!que.empty())
    {
        int sx = que.front();
        que.pop();
        vis[sx] = false;
        for(int i = head[sx]; i != -1; i = f[i].next)
        {
            int v = f[i].v;
            if(dis[v] > dis[sx]+f[i].w)
            {
                dis[v] = dis[sx]+f[i].w;
                if(!vis[v])
                {
                    vis[v] = true;
                    que.push(v);
                }
            }
        }
    }
    int Max = 0;
    for(int i = 1; i <= n; i++)
    {
        if(!dis[i]) continue;
        Max = max(Max, dis[i]);
    }
    return Max;
}
vector<int>g[maxn];
bool sta[maxn];

void dfs(int x, int fa)
{
    sta[x] = 1;
    for(int i = head[x]; i != -1; i = f[i].next)
    {
        int sx = f[i].v;
        if(sta[sx] || sx == fa) continue;
        dfs(sx, x);
    }
}

int main()
{
    int n;
    while(~scanf("%d",&n) && n)
    {
        for(int i = 1; i <= n; i++)
        {
            cin >>str;
            mp[str] = i;
        }
        int m;
        scanf("%d",&m);
        int x, y;
        init();
        for(int i = 0; i < m; i++)
        {
            cin >>str>>str1;
            x = mp[str];
            y = mp[str1];
            add(x, y, 1);
        }
        memset(sta, false, sizeof(sta));
        dfs(1, -1);
        int xflag = 0;
        for(int i = 1; i <= n; i++)
        {
            if(sta[i]) continue;
            xflag = 1;
            break;
        }
        if(xflag)
        {
            puts("-1");
            continue;
        }
        int flag = 0;
        int Max = 0;
        for(int i = 1; i <= n; i++)
        {
            int sx = spfa(i, n);
            if(sx >= 7)
            {
                flag = 1;
                break;
            }
            Max = max(Max, sx);
        }
        if(flag)
        {
            puts("-1");
            continue;
        }
        printf("%d\n",Max);
    }
}

HDU 4460 Friend Chains(map + spfa)

原文：http://blog.csdn.net/xu12110501127/article/details/41592819