网络流EK算法洛谷P3376 day4

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 5e3 + 5;
ll n, m, s, t, ee = 2;
int flg[1000][1000], vis[1000], h[1000];
struct edge{
    ll v, next, val;
}e[N << 1];

void addedge(ll u, ll v, ll w)
{
    e[ee] = (edge){v, h[u], w};
    h[u] = ee++;
}

struct Pre{
    ll v, e_id;
}pre[N];

ll bfs()
{
    memset(vis, 0, sizeof(vis));
    //memset(pre, -1, sizeof(pre));
    vis[s] = 1;
    queue<int>q;
    q.push(s);
    while(q.size())
    {
        ll u = q.front();
        q.pop();
        for(ll i = h[u]; i; i = e[i].next)
        {
            ll v = e[i].v;
            if(!vis[v] && e[i].val)
            {
                pre[v].v = u;
                pre[v].e_id = i;
                if(v == t)return 1;
                vis[v] = 1;
                q.push(v);
            }
        }
    }
    return 0;
}
ll EK()
{
    ll ans = 0;
    while(bfs())
    {
        ll mi = INT_MAX;
        for(int i = t; i != s; i = pre[i].v)mi = min(mi, e[pre[i].e_id].val);
        for(int i = t; i != s; i = pre[i].v)
        {
            e[pre[i].e_id].val -= mi;
            e[pre[i].e_id ^ 1].val += mi;
        }
        ans += mi;
    }
    return ans;
}
int main()
{
    ios::sync_with_stdio(false); cin.tie(0);
    //freopen("P3376_9.in","r",stdin);    
    cin >> n >> m >> s >> t;
    ll u, v, w;
    for(int i = 1; i <= m; i++)
    {
        cin >> u >> v >> w;
        if(!flg[u][v])
        {
            flg[u][v] = ee;
            addedge(u, v, w);
            addedge(v, u, 0);
        }
        else e[flg[u][v]].val += w;
    }
    cout << EK() <<"\n";
    return 0;    
}