Time Limit: 2000MS | Memory Limit: 65536K | |
Total Submissions: 83995 | Accepted: 31141 |
Description
While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way path that delivers you to its destination at a time that is BEFORE you entered the wormhole! Each of FJ's farms comprises N (1 ≤ N ≤ 500) fields conveniently numbered 1..N, M (1 ≤ M ≤ 2500) paths, and W (1 ≤ W ≤ 200) wormholes.
As FJ is an avid time-traveling fan, he wants to do the following: start at some field, travel through some paths and wormholes, and return to the starting field a time before his initial departure. Perhaps he will be able to meet himself :) .
To help FJ find out whether this is possible or not, he will supply you with complete maps to F (1 ≤ F ≤ 5) of his farms. No paths will take longer than 10,000 seconds to travel and no wormhole can bring FJ back in time by more than 10,000 seconds.
Input
Line 1: A single integer, F. F farm descriptions follow.Line 1 of each farm: Three space-separated integers respectively: N, M, and W
Lines 2..M+1 of each farm: Three space-separated numbers (S, E, T) that describe, respectively: a bidirectional path between S and E that requires T seconds to traverse. Two fields might be connected by more than one path.
Lines M+2..M+W+1 of each farm: Three space-separated numbers (S, E, T) that describe, respectively: A one way path from S to E that also moves the traveler back T seconds.
Output
Lines 1..F: For each farm, output "YES" if FJ can achieve his goal, otherwise output "NO" (do not include the quotes).Sample Input
2 3 3 1 1 2 2 1 3 4 2 3 1 3 1 3 3 2 1 1 2 3 2 3 4 3 1 8
Sample Output
NO YES
Hint
For farm 1, FJ cannot travel back in time.For farm 2, FJ could travel back in time by the cycle 1->2->3->1, arriving back at his starting location 1 second before he leaves. He could start from anywhere on the cycle to accomplish this. 题目大意: 有n(1 ≤ n ≤ 500)个农场,m(1 ≤ m ≤ 2500)条路, w(1 ≤ w ≤ 200)个虫洞,虫洞是单向的,而路径始终是双向的(例如有(5, 6), 则必有(6, 5))。 问农夫是否能够通过一些路与虫洞,使得农夫能够看到以前的自己(时间倒流)。 思路:把每个顶点都入一次队,判断图中是否存在负环即可。 之前把重复的边也处理了,导致了WA多发(太惨了 删掉以后就A了 代码:
#include <iostream> #include <queue> #include <cstring> #include <vector> #include <set> #include <algorithm> #include <string> #include <cstdio> #include <cstdlib> using namespace std; #define MAXN 2503 #define INF 0x3f3f3f3f int dis[MAXN]; int n, m, wei; int h[4 * MAXN], w[4 * MAXN], e[4 * MAXN], ne[4 * MAXN], cnt[MAXN], idx; bool vis[MAXN]; inline int read() { int s = 0, w = 1; char ch = getchar(); while (ch<'0' || ch>'9') { if (ch == '-')w = -1; ch = getchar(); } while (ch >= '0'&&ch <= '9') s = s * 10 + ch - '0', ch = getchar(); return s * w; } void add(int u, int v, int wei){ e[idx] = v; w[idx] = wei; ne[idx] = h[u]; h[u] = idx ++; } bool spfa(){ memset(cnt, 0, sizeof cnt); memset(vis, 0, sizeof vis); int cur; queue<int> q; for(int i = 1; i <= n; ++ i){ q.push(i); vis[i] = true; cnt[i] ++; } while(!q.empty()){ cur = q.front(); q.pop(); vis[cur] = false; for(int i = h[cur]; i != -1; i = ne[i]){ int j = e[i]; if(dis[j] > dis[cur] + w[i]){ dis[j] = dis[cur] + w[i]; cnt[j] ++; if(cnt[j] > n) return true; if(!vis[j]){ q.push(j); vis[j] = true; } } } } return false; } int main(){ int t, S, E, T, W; for(cin >> t; t --;){ idx = 0; memset(h, -1, sizeof h); cin >> n >> m >> W; for(int i = 0; i < m; ++ i){ S = read(); E = read(); wei = read(); add(S, E, wei); add(E, S, wei); } for(int i = 0; i < W; ++ i){ S = read(); E = read(); wei = read(); add(S, E, -wei); } cout << (spfa()?"YES":"NO") << endl; } return 0; }