poj 1789 Truck History MST(最小生成树)

//poj 1789 Truck History
//MST(minimum spanning tree)
//It's mean is: there is several type of truck and every truck
//is marked by a string with 7 lowercase character
//and distance between two truck is the number of differnt character
//between two string

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

#define N 2005
#define INF 1 << 30

int n;
int map[N][N], dis[N], ans;
char type[N][9];
bool vis[N];

void prim()
{
    for(int i = 0; i < n; ++i)
    {
        dis[i] = INF;
        vis[i] = false;
    }
    dis[0] = 0;
    while(1)
    {
        int now = -1, min = INF;
        for(int i = 0; i < n; ++i)
        {
            if(min > dis[i] && vis[i] == false)
            {
                min = dis[i];
                now = i;
            }
        }
        if(now == -1)
            break;
        ans += min;         //accumulate the answer
        vis[now] = true;    //have been visited
        for(int i = 0; i < n; ++i)
            if(vis[i] == false && dis[i] > map[now][i])
                dis[i] = map[now][i];
    }
}

int main()
{
    //freopen("in.txt", "r", stdin);
    while(scanf("%d", &n), n)
    {
        ans = 0;
        memset(map, 0, sizeof(map));
        for(int i = 0; i < n; ++i)
        {
            scanf("%s", type[i]);
            for(int j = 0; j < i; ++j)
            {
                int dist = 0;
                for(int k = 0; k < 7; ++k)
                    if(type[i][k] != type[j][k])
                        dist++;
                map[i][j] = map[j][i] = dist;   //build map
            }
        }
        prim();
        printf("The highest possible quality is 1/%d.\n", ans);
    }
    return 0;
}