LGOJ P3385 模板负环（纯属娱乐）

Posted 2020-11-30 kion

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;
}