LGOJ P3385 【【模板】负环】（纯属娱乐）

SPjkstra算法

SPjkstra算法，顾名思义就是SPFA与Dijkstra算法重重叠加，效果良好。

SPjkstra算法：193ms

SPFA算法：13266ms

说明：为第一个测试点的测试数据。

什么是SPjkstra算法？

就是某些蒟蒻比如说我因为只学了SPFA算法，但是发现程序运行太慢了，直接将SPFA中的队列换成优先队列的偷懒做法。

// P3385.cpp: 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <cmath>
#include <queue>
#include <list>
#include <memory.h>
using namespace std;
int n, m;
const int maxn = 20005;
struct edge
{
	int to, w;
	edge()
	{
		to = 0;
		w = 0;
	}
	edge(int a, int b)
	{
		to = a;
		w = b;
	}
};
vector< vector<edge> > G;
int d[maxn];
const int INF = 1 << 30;
int cnt[maxn];
bool flag = 1;
bool v[maxn];
inline int read()
{
	bool minus = 0;
	int x = 0; char c = getchar();
	for (; !isdigit(c); c = getchar())
		if (c == '-')minus = 1;
	for (; isdigit(c); c = getchar())
		x = x * 10 + c - '0';
	if (minus)return -x;
	return x;
}

inline void intial()//初始化函数
{
	G.clear();
	memset(v, 0, sizeof(v));
	for (int i = 0; i < maxn; i++)
		d[i] = INF;
	memset(cnt, 0, sizeof(cnt));
}

struct node//只是为了重载符号
{
	int x;
	node(int a)
	{
		x = a;
	}
	bool operator <(const node& i)const
	{
		return this->x > i.x;
	}
};

void SPijkstra(int start)//本来是SPFA
{
	priority_queue<node> Q;
	//queue<node> Q;
	Q.push(node(start));
	v[start] = 1;
	d[start] = 0;
	cnt[start] = 1;
	while (!Q.empty())
	{
		int x = Q.top().x;
		Q.pop();
		v[x] = 0;
		for (int i = 0; i < G[x].size(); i++)
		{
			int t = G[x][i].to, w = G[x][i].w;
			if (d[x] + w < d[t])
			{
				d[t] = d[x] + w;
				cnt[t] = cnt[x] + 1;

				if (!v[t])
				{
					if (cnt[t] > n)//BFS深度超过n == 存在负环
					{
						flag = 0;
						return;
					}
					v[t] = 1;
					Q.push(node(t));
				}
			}
		}
	}
}

int main()
{
	freopen("C:\\Users\\Administrator\\Downloads\\testdata (16).in", "r", stdin);
	int oo;
	oo = read();
	for (int pp = 1; pp <= oo; pp++)
	{
		flag = 1;
		intial();
		n = read();
		m = read();
		vector<edge>samp;
		G.assign(n + 10, samp);
		for (int i = 1; i <= m; i++)
		{
			int a, b, w;
			a = read();
			b = read();
			w = read();
			if (w < 0)
			{
				G[a].push_back(edge(b, w));
			}
			else
			{
				G[a].push_back(edge(b, w));
				G[b].push_back(edge(a, w));
			}
		}

		SPijkstra(1);

		if (flag == 0)
			cout << "YE5" << endl;
		if (flag)
			cout << "N0" << endl;
	}
	return 0;
}