poj Is the Information Reliable?

差分约系统。
至于等于就大于等于且小于等于。
发现以前的spfa板子有问题，点要提前出队，才能判自环。
code：

#include<cstdio>#include<cstdlib>#include<cstring>#include<iostream>#include<queue>#include<algorithm>using namespace std;struct node{    int x,y,c,next;}a[2110000];int len=0,last[500010];int n,m;void ins(int x,int y,int c){    a[++len].x=x;a[len].y=y;a[len].c=c;    a[len].next=last[x];last[x]=len;}queue<int> q;int d[500010],ru[500010];bool u[500010];bool spfa(){    memset(d,0,sizeof(d));    memset(ru,0,sizeof(ru));    memset(u,true,sizeof(u));    for(int i=0;i<=n;i++) q.push(i);    while(!q.empty())    {        int x=q.front();u[x]=false;        for(int i=last[x];i;i=a[i].next)        {            int y=a[i].y;            if(d[x]+a[i].c>d[y])            {                d[y]=d[x]+a[i].c;                ru[i]++;if(ru[i]>n) return false;                if(!u[y]) u[y]=true,q.push(y);            }        }        q.pop();    }    return true;}int main(){    while(scanf("%d %d",&n,&m)!=EOF)    {        memset(last,0,sizeof(last));len=0;        for(int i=1;i<=m;i++)        {            char s[5];            scanf("%s",s+1);            if(s[1]=='P')            {                int x,y,c;scanf("%d %d %d",&x,&y,&c);                ins(x,y,c);ins(y,x,-c);            }            else            {                int x,y;scanf("%d %d",&x,&y);                ins(x,y,1);            }        }        if(spfa()) printf("Reliable\n");        else printf("Unreliable\n");    }}