SAP解决USACO4.2.1网络流问题

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/*ID: hk945801TASK: ditchLANG: C++*/#include<iostream>#include<cstdio>#include<cstring>using namespace std;int a[201][201],pre[201],level[201],gap[201];int main(){    int i,j,k,m,n;    freopen("ditch.in","r",stdin);    freopen("ditch.out","w",stdout);    cin>>m>>n;    for(i=1;i<=m;i++){    int x,y,z;        cin>>x>>y>>z;        a[x][y]+=z;    }    int s=1;    gap[0]=n;    int v,p=pre[s]=s;int ans=0,sum;    while(level[s]<n){        for(v=1;v<=n;v++){            if(a[p][v]>0 && level[p]==level[v]+1){                break;            }        }        if(v<=n){            pre[v]=p;            p=v;            if(v==n){                sum=1000000001;                for(i=v;i!=s;i=pre[i]){                    if(sum>a[pre[i]][i])sum=a[pre[i]][i];                }                ans+=sum;                for(i=v;i!=s;i=pre[i]){                    a[pre[i]][i]-=sum;                    a[i][pre[i]]+=sum;                }                p=s;            }        }else {            int min=n;            for(v=1;v<=n;v++){                if(a[p][v]>0 && min>level[v]){                    min=level[v];                }            }            gap[level[p]]--;            if(gap[level[p]]==0)break;            level[p]=min+1;            gap[level[p]]++;            p=pre[p];        }    }    cout<<ans<<endl;    return 0;}
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