Question

I am trying to solve this Problem on UVA.The question is about finding the max-flow in the graph.I used Edmond-karp algorithm but I am continuously getting wrong answer.Can any one tell me what's wrong in my code ? My code :

#include<bits/stdc++.h>
using namespace std;
#define MX 1000000007
#define LL long long
#define ri(x) scanf("%d",&x)
#define rl(x) scanf("%lld",&x)
#define len(x) x.length()
#define FOR(i,a,n) for(int i=a;i<n;i++)
#define FORE(i,a,n) for(int i=a;i<=n;i++)
template<class T1> inline T1 maxi(T1 a,T1 b){return a>b?a:b;}
template<class T2> inline T2 mini(T2 a,T2 b){return a<b?a:b;}
int parent[101],G[101][101],rG[101][101];
bool bfs(int s,int t,int n)
{   
    bool vis[n+2];
    memset(parent,0,sizeof parent);
    memset(vis,0,sizeof vis);
    queue<int>Q;
    Q.push(s);
    vis[s]=true;
    while(!Q.empty())
    {
        int fnt=Q.front();
        Q.pop();
        for(int v=1;v<=n;v++)
        {
            if(!vis[v] and G[fnt][v]>0)
            {
                vis[v]=true;
                parent[v]=fnt;
                Q.push(v);
            }
        }
    }
    return vis[t];
}
int main()
{
    int n,tst=1;
    ri(n);
    while(n)
    {
        int s,t,c,flow=0;
        ri(s),ri(t),ri(c);
        FORE(i,1,c)
        {
            int x,y,z;
            ri(x),ri(y),ri(z);
            G[x][y]+=z;
            G[y][x]+=z;
        }
        while(bfs(s,t,n))
        {
            int path=9999999;
            for(int v=t;v!=s;v=parent[v])
            {
                int u=parent[v];
                path=mini(path,G[u][v]);
            }
            for(int v=t;v!=s;v=parent[v])
            {
                int u=parent[v];
                G[u][v]-=path;
                G[v][u]+=path;
            }
            flow+=path;
        }       
        printf("Network %d\nThe bandwidth is %d.\n\n", tst++, flow);
        ri(n);
    }
}

Answer 1

You push flow the other way around:

G[u][v]-=path;
G[v][u]+=path;

This should be:

G[u][v] += path;
G[v][u] -= path;

Also, I'm not sure about this part:

if(!vis[v] and G[fnt][v]>0)

[...]

path=mini(path,G[u][v]);

Because you are also allowed to take paths on which the flow is negative. You should not change G, which seems to be your capacities graph. Instead, you should have a matrix F that stores how much flow you send. Then your two conditions should be changed to:

if (!vis[v] && G[fnt][v] != F[fnt][v])

[...]

path = mini(path, G[u][v] - F[u][v])

And push flow on F, not G.

You seem to have thought about this since you declared a matrix rG, but you're never using it.

There might be other issues too. It's hard to tell without knowing what problems you're seeing.

UVA(820):Internet Bandwidth getting wrong answer?

1 个答案: