//ISAP+GAP優化+弧優化
#include <bits/stdc++.h>
#define H cout<<"HYX"<<endl;
using namespace std;
const int INF = 0x7f7f7f7f;
struct Edge{int from, to, f;};

int n, m, s, t, ct=1;
int Hed[10005], Nex[2*100005], Cur[10005], Dep[10005], Num[10005], Pre[10005];
Edge E[2*100005];

void Add(int &a, int &b, int &f){
    E[++ct].from=a, E[ct].to=b, E[ct].f=f, Nex[ct]=Hed[a], Hed[a]=ct;
    E[++ct].from=b, E[ct].to=a, E[ct].f=0, Nex[ct]=Hed[b], Hed[b]=ct;
}
void BFS(){
    queue<int> Q;
    memset(Dep, 0, sizeof Dep);
    Dep[t] = 1;  Q.push(t);
    int k;
    while(!Q.empty()){
        k = Q.front(); Q.pop();
        Num[Dep[k]]++;
        for(int i=Hed[k]; i; i=Nex[i]){
            if(E[i^1].f && !Dep[E[i].to]){
                Dep[E[i].to] = Dep[k]+1;
                Q.push(E[i].to);
            }
        }
    }
}
int Agument(){
    int k = t, flow = INF;
    while(k != s){
        if(E[Pre[k]].f < flow) flow = E[Pre[k]].f;
        k = E[Pre[k]].from;
    }
    k = t;
    while(k != s){
        E[Pre[k]].f -= flow;
        E[Pre[k]^1].f += flow;
        k = E[Pre[k]].from;
    }
    return flow;
}
int ISAP(int maxdep){
    int flow = 0, k = s, mindep;
    BFS();
    memcpy(Cur, Hed, sizeof Hed);
    bool can;
    while(Dep[s] <= maxdep){
        if(k == t){
            flow += Agument();
            k = s;
        }
        can = 0;
        for(int i=Cur[k]; i; i=Nex[i]){
            if(E[i].f && Dep[E[i].to]+1==Dep[k]){
                can = 1;
                Pre[E[i].to] = i;
                Cur[k] = i;
                k = E[i].to;
                break;
            }
        }
        if(!can){
            mindep = n+1;
            for(int i=Hed[k]; i; i=Nex[i])
                if(Dep[E[i].to]<mindep && E[i].f)
                    mindep = Dep[E[i].to];

            if(!--Num[Dep[k]]) break;
            Num[Dep[k]=mindep+1]++;
            Cur[k] = Hed[k];
            if(k != s) k = E[Pre[k]].from;
        }
    }
    return flow;
}
int main(){
    scanf("%d%d%d%d", &n, &m, &s, &t);
    int u, v, c;
    for(int i=1; i<=m; ++i){
        scanf("%d%d%d", &u, &v, &c);
        Add(u, v, c);
    }
    printf("%d", ISAP(n));
    return 0;
}
 

模板 ISAP網絡流+GAP優化+弧優化