POJ - 1847Tram最短路

Tram

Submit Status

Description

Tram network in Zagreb consists of a number of intersections and rails connecting some of them. In every intersection there is a switch pointing to the one of the rails going out of the intersection. When the tram enters the intersection it can leave only in
the direction the switch is pointing. If the driver wants to go some other way, he/she has to manually change the switch.

When a driver has do drive from intersection A to the intersection B he/she tries to choose the route that will minimize the number of times he/she will have to change the switches manually.

Write a program that will calculate the minimal number of switch changes necessary to travel from intersection A to intersection B.

Input

The first line of the input contains integers N, A and B, separated by a single blank character, 2 <= N <= 100, 1 <= A, B <= N, N is the number of intersections in the network, and intersections are numbered from 1 to N.

Each of the following N lines contain a sequence of integers separated by a single blank character. First number in the i-th line, Ki (0 <= Ki <= N-1), represents the number of rails going out of the i-th intersection. Next Ki numbers represents the intersections
directly connected to the i-th intersection.Switch in the i-th intersection is initially pointing in the direction of the first intersection listed.

Output

The first and only line of the output should contain the target minimal number. If there is no route from A to B the line should contain the integer "-1".

Sample Input

3 2 1
2 2 3
2 3 1
2 1 2

Sample Output

0

/*
 *Author: 2486
 *Memory: 716 KB    Time: 16 MS
 *Language: G++     Result: Accepted
 */
//理解题目意思，看懂题目即可
//其中注意细节,开关最开始的方向是由第一个数的值所确定的，所以对于第一个数的
//状态是不需要进行开关操作的
#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
using namespace std;
const int INF = 0x3f3f3f3f;
const int N = 100 + 5;
int n,a,b,x,y;
int d[N][N];

int warshall_floyd() {
    for(int  i = 0 ; i <= n; i ++) {
        d[i][i] = 0;
    }
    for(int k = 1; k <= n; k ++) {
        for(int i = 1 ; i <= n; i ++) {
            for(int j = 1; j <= n ; j ++) {
                d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
            }
        }
    }
}

int main() {
    while(~scanf("%d%d%d",&n,&a,&b)) {
        memset(d,0x3f,sizeof(d));
        for(int i = 1; i <= n; i ++) {
            scanf("%d",&x);
            for(int j = 0; j < x; j ++) {
                scanf("%d",&y);
                if(j) d[i][y] = 1;
                else  d[i][y] = 0;
            }
        }
        warshall_floyd();
        if(d[a][b] != INF)printf("%d\n",d[a][b]);
        else printf("-1\n");
    }
    return 0;
}