Bellman-Ford算法

本文主要是介绍Bellman-Ford算法，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

落谷题目p1993

#include<iostream>
#include<cstdio>
using namespace std;
typedef struct item
{
    /* data */
    int u;
    int v;
    int w;
}Item;
Item a[5000+5];
int n,m;
int p;
int dis[5000+5];
int main(){
    // cin >> n >> m;
    scanf("%d%d",&n,&m);
    int index = 0;
    for(int i=2;i<=n;i++){
        dis[i] = 1e9+7;
    }
    for(int i = 1;i<=m;i++){
        // cin >> p;
        scanf("%d",&p);
        index++;
        if(p==1){
            // cin >> a[index].v >> a[index].u;
            scanf("%d%d",&a[index].v ,&a[index].u);
            int ans;
            // cin >> ans;
            scanf("%d",&ans);
            a[index].w = -ans;
        }else if(p==2){
            // cin >> a[index].u >> a[index].v >> a[index].w;
            scanf("%d%d%d",&a[index].u,&a[index].v,&a[index].w);
        }else if(p==3){
            int x,y;
            // cin >> x >> y;
            scanf("%d%d",&x,&y);
            a[index].u = x;
            a[index].v = y;
            a[index].w = 0;
            index++;
            a[index].u = y;
            a[index].v = x;
            a[index].w = 0;
        }
    }
    int u,v,w;
    bool flag = false;
    bool opt = false;
    for(int k=1;k<=n-1;k++){
        flag = false;
        for(int i=1;i<=index;i++){
            u = a[i].u;
            v = a[i].v;
            w = a[i].w;
            if(dis[v]>dis[u]+w){
                dis[v] = dis[u] + w;
                flag = true;
            }
        }
        if(!flag){
            opt = true;
             break;
        }
    }
    if(opt){
     cout << "Yes";
     return 0;
    }
    flag = false;
    for(int i=1;i<=index;i++){
        u = a[i].u;
        v = a[i].v;
        w = a[i].w;
        if(dis[v]>dis[u]+w){
            dis[v] = dis[u] + w;
            flag = true;
            break;
        }
    }
    if(flag) cout << "No";
    else cout << "Yes";
    return 0;
}

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