POJ3268 Silver Cow Party(dijkstra)

Silver Cow Party

Time Limit: 2000MS		Memory Limit: 65536K
Total Submissions: 15644		Accepted: 7106

Description

One cow from each of N farms (1 ≤ N ≤ 1000) conveniently numbered 1..N is going to attend the big cow party to be held at farm #X (1 ≤ X ≤ N). A total of M (1 ≤ M ≤ 100,000) unidirectional (one-way roads connects pairs of farms; road i requires T_i (1 ≤ T_i ≤ 100) units of time to traverse.

Each cow must walk to the party and, when the party is over, return to her farm. Each cow is lazy and thus picks an optimal route with the shortest time. A cow's return route might be different from her original route to the party since roads are one-way.

Of all the cows, what is the longest amount of time a cow must spend walking to the party and back?

Input

Line 1: Three space-separated integers, respectively: N, M, and X 
Lines 2..M+1: Line i+1 describes road i with three space-separated integers: A_i, B_i, and T_i. The described road runs from farm A_i to farm B_i, requiring T_i time units to traverse.

Output

Line 1: One integer: the maximum of time any one cow must walk.

Sample Input

4 8 2
1 2 4
1 3 2
1 4 7
2 1 1
2 3 5
3 1 2
3 4 4
4 2 3

Sample Output

10

Hint

Cow 4 proceeds directly to the party (3 units) and returns via farms 1 and 3 (7 units), for a total of 10 time units.

一头牛可以从n个farm中任意一个出发到farm X开party，m条单向路，代价是t，问你去party和回farm最大和是多少。

两次dijkstra即可。

AC代码：

#include "iostream"
#include "cstdio"
#include "cstring"
#include "algorithm"
using namespace std;
const int inf = 0x3f3f3f3f;
const int maxn = 1010;
int map[maxn][maxn], d[maxn], dback[maxn];
bool vis[maxn];
int n, m, x;

int dijkstra()
{
	int v, mi;
	for(int i = 1; i <= n; ++i) {
		vis[i] = 0;
		d[i] = map[x][i];
		dback[i] = map[i][x];
	}
	for(int i = 1; i <= n; ++i) {
		mi = inf;
		for(int j = 1; j <= n; ++j) 
			if(!vis[j] && d[j] < mi) {
				v = j;
				mi = d[j];
			}
		vis[v] = true;
		for(int j = 1; j <= n; ++j)
			if(!vis[j] && map[v][j] + d[v] < d[j])
				d[j] = map[v][j] + d[v];
	}
	memset(vis, false, sizeof(vis));
	for(int i = 1; i <= n; ++i) {
		mi = inf;
		for(int j = 1; j <= n; ++j)
			if(!vis[j] && dback[j] < mi) {
				v = j;
				mi = dback[j];
			}
		vis[v] = true;
		for(int j = 1; j <= n; ++j)
			if(!vis[j] && map[j][v] + dback[v] < dback[j])
				dback[j] = map[j][v] + dback[v];
	}
	mi = -1;
	for(int i = 1; i <= n; ++i)
		if(d[i] + dback[i] > mi)
			mi = d[i] + dback[i];
	return mi;
}
int main(int argc, char const *argv[])
{
	int a, b, c;
	while(cin >> n >> m >> x) {
		for(int i = 1; i <= n; ++i)
			for(int j = 1; j <= n; ++j)
				if(i != j) map[i][j] = inf;
				else map[i][j] = 0;
		for(int i = 1; i <= m; ++i) {
			cin >> a >> b >> c;
			map[a][b] = c;
		}
		cout << dijkstra() << endl;
	}
	return 0;
}