携程编程大赛决赛-1004-最短路径的代价

这个题题意就不用说了，想了半天。

思路就是把所有的最短路径边求出来，然后再以删掉该边的花费作为代价建图，设立源点和汇点，求最大流即可。

代码：

#include<cstdio>
#include<cstring>
#include<iostream>
#include<queue>
using namespace std;
const int maxn=2000;
const int maxm=50000;
const int inf=1<<29;
struct Node
{
    int u;
    int v;
    int w;
    int c;
}s[maxn*maxn];
struct Snode
{
    int u,v;
}ee[maxn*maxn];
int x,y,n,m,e,des,st,head[maxn],nxt[maxm],cost[maxm],pnt[maxm],dist[maxn];
int we,whead[maxn],wpnt[maxm],wnxt[maxm],flow[maxm],level[maxn];
int f[maxn][maxn];
bool vis[maxn];
queue<int> q;
void AddEdge(int u,int v,int c)
{
    pnt[e]=v;nxt[e]=head[u];cost[e]=c;head[u]=e++;
    pnt[e]=u;nxt[e]=head[v];cost[e]=c;head[v]=e++;
}
int Spfa(int st,int des)
{
    for(int i=0;i<=n;i++)
	dist[i]=inf;
    dist[st]=0;
    while(!q.empty())
	q.pop();
    q.push(st);
    while(!q.empty())
    {
	int u=q.front();
	q.pop();
	vis[u]=0;
	for(int i=head[u];i!=-1;i=nxt[i])
	{
	    int v=pnt[i];
	    if(dist[v]>dist[u]+cost[i])
	    {
		dist[v]=dist[u]+cost[i];
		if(!vis[v])
		{
		    q.push(v);
		    vis[v]=1;
		}
	    }
	}
    }
    return dist[des];
}
void AddEdgeW(int u,int v,int f)
{
    wpnt[we]=v;wnxt[we]=whead[u];flow[we]=f;whead[u]=we++;
    wpnt[we]=u;wnxt[we]=whead[v];flow[we]=0;whead[v]=we++;
}
bool BFS(int st)
{
    memset(level,0,sizeof(level));
    while(!q.empty())
	q.pop();
    q.push(st);
    level[st]=1;
    while(!q.empty())
    {
	int u=q.front();
	q.pop();
	if(u==des)
	    return true;
	for(int i=whead[u];i!=-1;i=wnxt[i])
	    if(flow[i]&&!level[wpnt[i]])
	    {
		level[wpnt[i]]=level[u]+1;
		q.push(wpnt[i]);
	    }
    }
    return level[des];
}
int DFS(int u,int sum)
{
    if(u==des)
	return sum;
    for(int i=whead[u],t;i!=-1;i=wnxt[i])
    {
	if(flow[i]&&level[wpnt[i]]==level[u]+1&&(t=DFS(wpnt[i],min(sum,flow[i]))))
	{
	    flow[i]-=t;
	    flow[i^1]+=t;
	    return t;
	}
    }
    return level[u]=0;
}
int maxflow()
{
    int ans=0;
    while(BFS(st))
    {
	while(1)
	{
	    int val=DFS(st,inf);
	    if(!val)
		break;
	    ans+=val;
	}
    }
    return ans;
}
int main()
{
    while(scanf("%d%d",&n,&m)&&(n||m))
    {
	memset(head,-1,sizeof(head));
	e=we=0;
	scanf("%d%d",&x,&y);
	for(int i=0;i<m;i++)
	{
	    scanf("%d%d%d%d",&s[i].u,&s[i].v,&s[i].w,&s[i].c);
	    AddEdge(s[i].u,s[i].v,s[i].w);
	}
	int ans=Spfa(x,y);
	memset(whead,-1,sizeof(whead));
	memset(f,0,sizeof(f));
	int cnt=0;
	for(int i=0;i<m;i++)
	{
	    if(dist[s[i].u]+s[i].w==dist[s[i].v])
	    {
		if(!f[s[i].u][s[i].v])
		{
		    ee[cnt].u=s[i].u;
		    ee[cnt++].v=s[i].v;
		}
		f[s[i].u][s[i].v]+=s[i].c;
	    }
	    if(dist[s[i].v]+s[i].w==dist[s[i].u])
	    {
		if(!f[s[i].v][s[i].u])
		{
		    ee[cnt].u=s[i].v;
		    ee[cnt++].v=s[i].u;
		}
		f[s[i].v][s[i].u]+=s[i].c;
	    }
	}
	for(int i=0;i<cnt;i++)
	    AddEdgeW(ee[i].u,ee[i].v,f[ee[i].u][ee[i].v]);
	st=x,des=y;
	printf("%d\n",maxflow());
    }
    return 0;
}