POJ 2983 Is the Information Reliable?（差分约束）

题目链接：点击打开链接

题目大意：给定n个点，m条信息，信息包含两种，一种是P，给定u,v,w代表u点在v点的北方w光年处，一种是V，给定u,v代表u在v的北方至少1光年处，判断给定信息的真假。

题目解析：建立图，跑最长路，判断是不是有矛盾即正环，SPFA，要判断自环。

#include <algorithm>
#include <iostream>
#include <numeric>
#include <cstring>
#include <iomanip>
#include <string>
#include <vector>
#include <cstdio>
#include <bitset>
#include <queue>
#include <stack>
#include <cmath>
#include <map>
#include <set>
#define LL long long
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define Pair pair<int,int>
const int INF = 0x3f3f3f3f;
using namespace std;

struct node{
    int v,w,next;
}arr[200005];
int head[1005],tot,n,m;
int dis[1005],du[1005];
bool vis[1005];
char op[5];

void add_edge(int u,int v,int w){
    arr[tot].v = v;
    arr[tot].w = w;
    arr[tot].next = head[u];
    head[u] = tot++;
}

bool spfa(){
    queue<int>Q;
    for(int i=1;i<=n;i++){
        dis[i] = 0;
        vis[i] = true;
        du[i] = 1;
        Q.push(i);
    }
    while(!Q.empty()){
        int u = Q.front();Q.pop();
        if(du[u] > n){
            return false;
        }
        for(int i=head[u];i!=-1;i=arr[i].next){
            int v = arr[i].v;
            if(dis[v] < dis[u] + arr[i].w){
                dis[v] = dis[u] + arr[i].w;
                if(!vis[v]){
                    Q.push(v);
                    vis[v] = true;
                    du[v] += 1;
                }
            }
        }
        vis[u] = false;
    }
    return true;
}

int main(){
    while(~scanf("%d %d",&n,&m)){
        tot = 0;
        bool flag = false;
        memset(head,-1,sizeof(head));
        for(int i = 1,u,v,w;i <= m; i++){
            scanf("%s",op);
            if(op[0] == 'P'){
                scanf("%d %d %d",&u,&v,&w);
                if(u == v && w != 0){
                    flag = true;
                }
                add_edge(u,v,w);
                add_edge(v,u,-w);
            }
            else{
                scanf("%d %d",&u,&v);
                if(u == v){
                    flag = true;
                }
                add_edge(u,v,1);
            }
        }
        if(flag || !spfa())
            printf("Unreliable\n");
        else
            printf("Reliable\n");
    }
    return 0;
}