[AHOI2006]上学路线route

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题目
最短路图上最小割。
最短路图下为何不能直接用SPFA式Dinic。。。。会WA？WA一年了。

AC Code:

// luogu-judger-enable-o2
#include<bits/stdc++.h>
#define maxn 505
#define maxm 400005
#define inf 0x3f3f3f3f
using namespace std;

int n,m;
int info[maxn],Prev[maxm],to[maxm],cst[maxm],cap[maxm],cnt_e=1;
void Node(int u,int v,int c,int ct){ Prev[++cnt_e]=info[u],info[u]=cnt_e,to[cnt_e]=v,cap[cnt_e]=c,cst[cnt_e]=ct; }
void Line(int u,int v,int c,int ct){ Node(u,v,c,ct),Node(v,u,0,ct); }
int dis[maxn],ans;

int x[maxm],y[maxm],w[maxm],t[maxm];

bool inq[maxn];

int aug(int now,int Max)
{
    if(now == n) return Max;
    int inc,st=Max;
    for(int i=info[now];i;i=Prev[i])
        if(cap[i] && dis[to[i]]+1==dis[now])
        {
            inc = aug(to[i],min(cap[i],st));
            if(inc)st -= inc , cap[i] -= inc , cap[i^1] += inc;
            else dis[to[i]]=0;
            if(!st) break;
        }
    return Max - st;
}

bool BFS()
{
	static queue<int>q;
	memset(dis,-1,sizeof dis);
	q.push(n),dis[n]=0;
	for(int now;!q.empty();)
	{
		now = q.front() , q.pop();
		for(int i=info[now];i;i=Prev[i])
			if(cap[i^1] && dis[to[i]]==-1)
			{
				dis[to[i]] = dis[now] + 1;
				q.push(to[i]);
			}
	}
	return dis[1] != -1;
}

int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d%d%d",&x[i],&y[i],&w[i],&t[i]);
        Line(x[i],y[i],t[i],w[i]);
    }
    queue<int>q;
    memset(dis,0x3f,sizeof dis);
    q.push(n);dis[n] = 0;
    for(int now;!q.empty();)
    {
        now = q.front() , q.pop() , inq[now] = 0;
        for(int i=info[now];i;i=Prev[i])
            if(dis[to[i]] > dis[now] + cst[i])
            {
                dis[to[i]] = dis[now] + cst[i];
                if(!inq[to[i]])
                    inq[to[i]]=1,q.push(to[i]);
            }
    }
    ans = dis[1];
	memset(info,0,sizeof info),cnt_e=1;
	for(int i=1;i<=m;i++)
	{
		if(dis[x[i]] == dis[y[i]] + w[i]) Line(x[i],y[i],t[i],0);
		if(dis[y[i]] == dis[x[i]] + w[i]) Line(y[i],x[i],t[i],0);
	}
    
    int stm = 0;
    for(;BFS();)
        stm += aug(1,inf);
    printf("%d\n%d\n",ans,stm);
}