[BZOJ1663] [Usaco2006 Open]赶集（spfa最长路）

传送门

按照时间t排序

如果 t[i] + map[i][j] <= t[j]，就在i和j之间连一条边

然后spfa找最长路

#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define N 401

using namespace std;

int n, ans, cnt;
int a[N][N], map[N][N], dis[N], head[N], to[N * 100], next[N * 100];
bool vis[N];
queue <int> q;

struct node
{
	int t, id;
}p[N];

inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -1;
	for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
	return x * f;
}

inline bool cmp(node x, node y)
{
	return x.t < y.t;
}

inline void spfa()
{
	int i, u, v;
	q.push(0);
	while(!q.empty())
	{
		u = q.front();
		q.pop();
		vis[u] = 0;
		for(i = head[u]; ~i; i = next[i])
		{
			v = to[i];
			if(dis[v] < dis[u] + 1)
			{
				dis[v] = dis[u] + 1;
				if(!vis[v])
				{
					q.push(v);
					vis[v] = 1;
				}
			}
		}
	}
}

inline void add(int x, int y)
{
	to[cnt] = y;
	next[cnt] = head[x];
	head[x] = cnt++;
}

int main()
{
	int i, j, k;
	n = read();
	memset(head, -1, sizeof(head));
	for(i = 1; i <= n; i++)
		p[i].id = i, p[i].t = read();
	for(i = 1; i <= n; i++)
		for(j = 1; j <= n; j++)
			map[i][j] = read();
	sort(p + 1, p + n + 1, cmp);
	for(i = 1; i <= n; i++) map[0][i] = map[1][i]; 
	p[0].id = p[0].t = 0; 
	for(i = 0; i <= n; i++)
		for(j = i + 1; j <= n; j++)
			if(p[i].t + map[p[i].id][p[j].id] <= p[j].t)
				add(p[i].id, p[j].id);
	spfa();
	for(i = 1; i <= n; i++) ans = max(ans, dis[i]);
	printf("%d
", ans);
	return 0;
}