Author: lllyouo
Date: 2025-04-06
tag: 差分约束
link: https://www.luogu.com.cn/problem/P1993

问题描述

link

分析

略

参考代码

#include <bits/stdc++.h>
using namespace std;

const int N = 5010, M = 2 * N;
int h[N], e[M], w[M], ne[M], idx;
int n, m;
int dist[N], vis[N], cnt[N];

void add(int a, int b, int c) {
    e[idx] = b;
    w[idx] = c;
    ne[idx] = h[a];
    h[a] = idx++;
}

bool spfa() {  
    queue<int> q;
    
    for (int i = 1; i <= n; i++) {
        q.push(i);
        vis[i] = 1;
    }

    while (!q.empty()) {
        int k = q.front();
        q.pop();
        vis[k] = 0;

        for (int i = h[k]; i != -1; i = ne[i]) {
            int j = e[i];
            if (dist[j] > dist[k] + w[i]) {
                dist[j] = dist[k] + w[i];
                cnt[j] = cnt[k] + 1;
                
                if (cnt[j] >= n) return true;
                if (!vis[j]) {
                    q.push(j);
                    vis[j] = 1;
                }
            }
        }
    }

    return false;
}

int main() {
    memset(h, -1, sizeof h);
    cin >> n >> m;
    for (int i = 1; i <= m; i++) {
        int o, a, b, c; cin >> o;
        if (o == 1) {
            cin >> a >> b >> c;
            add(a, b, -c);
        } else if (o == 2) {
            cin >> a >> b >> c;
            add(b, a, c);
        } else {
            cin >> a >> b;
            add(a, b, 0);
            add(b, a, 0);
        }
    }
    if (!spfa()) puts("Yes");
    else puts("No");

    return 0;
}