有編號為1－N的牛，它們之間存在一些單向的路徑。給定一頭牛的編號，其他牛要去拜訪它並且拜訪完之後要返回自己原來的位置，求這些牛中所花的最長的來回時間是多少。

每頭牛返回的最短時間很簡單就可以算出來，這相當於從目標牛為起點求單源最短路徑。但每頭牛出發到目標牛的最短時間無法直接算出來，稍微轉換一下，發現這個最短時間其實可以通過把所有的邊取反向，然後再從目標牛求一次單源最短路徑得到。得到這兩個最短路徑之後，取它們的和的最大者即可。

鄰接陣O(n^2)：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <climits>
using namespace std;

const int N = 1005;
int edge[N][N];
int n, e, x;
int mindis1[N];
int mindis2[N];
bool vis1[N];
bool vis2[N];

void init()
{
	for (int i = 0; i < n; ++i) 
	{
		for (int j = 0; j < n; ++j)
		{
			edge[i][j] = -1;
		}
	}
}

void dijkstra(int s)
{
	int pos1, pos2, te1, te2, tm1, tm2;
	for (int i = 0; i < n; ++i) 
	{
		vis1[i] = false;
		vis2[i] = false;
		mindis1[i] = 999999;
		mindis2[i] = 999999;
	}
	mindis1[s] = 0;
	mindis2[s] = 0;
	vis1[s] = true;
	vis2[s] = true;
	pos1 = s;
	pos2 = s;
	for (int i = 0; i < n - 1; ++i)
	{
		for (int j = 0; j < n; ++j)
		{
			if (!vis1[j] && edge[pos1][j] != -1 &&  mindis1[pos1] + edge[pos1][j] < mindis1[j])
			{
				mindis1[j] = mindis1[pos1] +edge[pos1][j];
			}
			if (!vis2[j] && edge[j][pos2] != -1 && mindis2[pos2] + edge[j][pos2] < mindis2[j])
			{
				mindis2[j] = mindis2[pos2] + edge[j][pos2];
			}
		}
		tm1 = 999999;
		tm2 = 999999;
		for (int j = 0; j < n; ++j)
		{
			if (!vis1[j] && mindis1[j] < tm1)
			{
				tm1 = mindis1[j];
				te1 = j;
			}
			if (!vis2[j] && mindis2[j] < tm2)
			{
				tm2 = mindis2[j];
				te2 = j;
			}
		}
		vis1[te1] = true;
		pos1 = te1;
		vis2[te2] = true;
		pos2 = te2;
	}
}

int main()
{
	int beg, end, dis;
	scanf("%d%d%d", &n, &e, &x);
	init();
	for (int i = 0; i < e; ++i)
	{
		scanf("%d%d%d", &beg, &end, &dis);
		--beg;
		--end;
		edge[beg][end] = dis;
	}
	dijkstra(--x);
	int ans = 0;
	for (int i = 0; i < n; ++i)
	{
		if (i == x) continue;
		if (mindis1[i] + mindis2[i] > ans) ans = mindis1[i] + mindis2[i];
	}
	printf("%d\n", ans);
	return 0;
}
 

鄰接表O(E*logE)：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <queue>
using namespace std;

const int N = 1005;
const int E = 1000005;
struct Edge
{
	int pnt;
	int dis;
	int next;
};
Edge edge1[E], edge2[E];
int cur1, cur2;
int neigh1[N], neigh2[N];
int n, e, x;
int mindis1[N];
int mindis2[N];
bool vis1[N];
bool vis2[N];
struct Qnode
{
	int pnt;
	int dis;
	Qnode(int _pnt, int _dis): pnt(_pnt), dis(_dis){}
	bool operator < (const Qnode& node) const
	{
		return dis > node.dis;
	}
};

void addedge(int beg, int end, int dis)
{
	edge1[cur1].pnt = end;
	edge1[cur1].dis = dis;
	edge1[cur1].next = neigh1[beg];
	neigh1[beg] = cur1;
	++cur1;

	edge2[cur2].pnt = beg;
	edge2[cur2].dis = dis;
	edge2[cur2].next = neigh2[end];
	neigh2[end] = cur2;
	++cur2;
}

void dijkstra(int s)
{
	int pre1, pre2, te1, te2, pnt1, pnt2;
	priority_queue<Qnode> pq1, pq2;
	for (int i = 0; i < n; ++i) 
	{
		vis1[i] = false;
		vis2[i] = false;
		mindis1[i] = 999999;
		mindis2[i] = 999999;
	}
	mindis1[s] = 0;
	mindis2[s] = 0;
	vis1[s] = true;
	vis2[s] = true;
	pq1.push(Qnode(s, 0));
	pq2.push(Qnode(s, 0));
	pre1 = s;
	pre2 = s;
	for (int i = 0; i < n - 1; ++i)
	{
		te1 = neigh1[pre1];
		while (te1 != -1)
		{
			pnt1 = edge1[te1].pnt;
			if (!vis1[pnt1] && mindis1[pre1] + edge1[te1].dis < mindis1[pnt1])
			{
				mindis1[pnt1] = mindis1[pre1] + edge1[te1].dis;
				pq1.push(Qnode(pnt1, mindis1[pnt1]));
			}
			te1 = edge1[te1].next;
		}
		while (!pq1.empty() && vis1[pq1.top().pnt]) pq1.pop();
		pre1 = pq1.top().pnt;
		vis1[pre1] = true;
		pq1.pop();

		te2 = neigh2[pre2];
		while (te2 != -1)
		{
			pnt2 = edge2[te2].pnt;
			if (!vis2[pnt2] && mindis2[pre2] + edge2[te2].dis < mindis2[pnt2])
			{
				mindis2[pnt2] = mindis2[pre2] + edge2[te2].dis;
				pq2.push(Qnode(pnt2, mindis2[pnt2]));
			}
			te2 = edge2[te2].next;
		}
		while (!pq2.empty() && vis2[pq2.top().pnt]) pq2.pop();
		pre2 = pq2.top().pnt;
		vis2[pre2] = true;
		pq2.pop();
	}
}

int main()
{
	int beg, end, dis;
	scanf("%d%d%d", &n, &e, &x);
	for (int i = 0; i < n; ++i) 
	{
		neigh1[i] = -1;
		neigh2[i] = -1;
	}
	cur1 = cur2 = 0;
	for (int i = 0; i < e; ++i)
	{
		scanf("%d%d%d", &beg, &end, &dis);
		--beg;
		--end;
		addedge(beg, end, dis);
	}
	dijkstra(--x);
	int ans = 0;
	for (int i = 0; i < n; ++i)
	{
		if (i == x) continue;
		if (mindis1[i] + mindis2[i] > ans) ans = mindis1[i] + mindis2[i];
	}
	printf("%d\n", ans);
	return 0;
}