Poj 3615 Cow Hurdles【Floyd】

Cow Hurdles

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 7404		Accepted: 3359

Description

Farmer John wants the cows to prepare for the county jumping competition, so Bessie and the gang are practicing jumping over hurdles. They are getting tired, though, so they want to be able to use as little energy as possible to jump over the hurdles.

Obviously, it is not very difficult for a cow to jump over several very short hurdles, but one tall hurdle can be very stressful. Thus, the cows are only concerned about the height of the tallest hurdle they have to jump over.

The cows' practice room has N (1 ≤ N ≤ 300) stations, conveniently labeled 1..N. A set of M (1 ≤ M ≤ 25,000) one-way paths connects pairs of stations; the paths are also conveniently labeled 1..M. Path i travels
from station Si to station Ei and contains exactly one hurdle of height Hi (1 ≤ Hi ≤ 1,000,000). Cows must jump hurdles in any path they traverse.

The cows have T (1 ≤ T ≤ 40,000) tasks to complete. Task i comprises two distinct numbers, Ai and Bi (1 ≤ Ai ≤ N; 1 ≤ Bi ≤ N), which
connote that a cow has to travel from station Ai to station Bi (by traversing over one or more paths over some route). The cows want to take a path the minimizes the height of the tallest hurdle they jump over when traveling
from Ai to Bi . Your job is to write a program that determines the path whose tallest hurdle is smallest and report that height.

　

Input

* Line 1: Three space-separated integers: N, M, and T

* Lines 2..M+1: Line i+1 contains three space-separated integers: Si , Ei , and Hi 

* Lines M+2..M+T+1: Line i+M+1 contains two space-separated integers that describe task i: Ai and Bi

Output

* Lines 1..T: Line i contains the result for task i and tells the smallest possible maximum height necessary to travel between the stations. Output -1 if it is impossible to travel between the two stations.

Sample Input

5 6 3

1 2 12

3 2 8

1 3 5

2 5 3

3 4 4

2 4 8

3 4

1 2

5 1

Sample Output

4

8

-1

Source

USACO 2007 November Silver

 

 题意：奶牛们为了比赛要刻苦训练跳木桩。现在有n个木桩，并知道其中m对木桩的高度差。问奶牛们能从木桩u跳到木桩v，最少的跳跃高度是多少.有t个询问。

思路：

1、

if（map【j】【i】！=03f3f3f3f&&map【i】【k】！=0x3f3f3f3f）

map【j】【k】=min（map【j】【k】，max（map【j】【i】，map【i】【k】））

2、

跑一遍Floyd即可。

Ac代码：

#include<stdio.h>
#include<string.h>
#include<iostream>
using namespace std;
int map[500][500];
int main()
{
int n,m,t;
while(~scanf("%d%d%d",&n,&m,&t))
{
memset(map,0x3f3f3f3f,sizeof(map));
for(int i=0;i<m;i++)
{
int x,y,w;
scanf("%d%d%d",&x,&y,&w);
map[x][y]=w;
}
for(int i=1;i<=n;i++)
{
for(int j=1;j<=n;j++)
{
for(int k=1;k<=n;k++)
{
if(map[j][i]!=0x3f3f3f3f&&map[i][k]!=0x3f3f3f3f)
map[j][k]=min(map[j][k],max(map[j][i],map[i][k]));
}
}
}
while(t--)
{
int x,y;
scanf("%d%d",&x,&y);
if(map[x][y]==0x3f3f3f3f)printf("-1\n");
else printf("%d\n",map[x][y]);
}
}
}