Flux maxim de cost minim SPFA(70p infoarena)

#include <bits/stdc++.h>

using namespace std;

ifstream fin("fmcm.in");
ofstream fout("fmcm.out");

int N, source, sink;
bool flag;
vector < vector < int > > Graph, Capacity, Cost;
vector < int > parent, D;

const int INF = 2e9;

inline void min_self(int& a, int b) {
    a = min(a, b);
}

int SPFA() {
    D.assign(N + 1, INF);
    parent.assign(N + 1, -1);
    D[source] = 0;
    queue < int > Q;
    Q.emplace(source);
    vector < bool > inQ(N + 1);
    inQ[source] = true;
    while(!Q.empty()) {
        int curr = Q.front();
        Q.pop();
        inQ[curr] = false;
        for(int next : Graph[curr])
            if(Capacity[curr][next] && D[curr] + Cost[curr][next] < D[next]) {
                D[next] = D[curr] + Cost[curr][next];
                parent[next] = curr;
                if(!inQ[next]) {
                    Q.emplace(next);
                    inQ[next] = true;
                }
            }
    }
    if(D[sink] != INF) {
        flag = true;
        int new_flow = INF;
        for(int node = sink; node != source; node = parent[node])
            min_self(new_flow, Capacity[parent[node]][node]);
        if(new_flow == 0)
            return 0;
        for(int node = sink; node != source; node = parent[node]) {
            Capacity[parent[node]][node] -= new_flow;
            Capacity[node][parent[node]] += new_flow;
        }
        return new_flow * D[sink];
    }
    return 0;
}

int min_cost_flow() {
    int ans = 0;
    flag = true;
    while(flag) {
        flag = false;
        ans += SPFA();
    }
    return ans;
}

int main() {
    fin.sync_with_stdio(false);
    fout.sync_with_stdio(false);
    fin.tie(nullptr);
    fout.tie(nullptr);
    int M;
    fin >> N >> M >> source >> sink;
    Graph.resize(N + 1);
    Capacity = Cost = vector < vector < int > >(N + 1, vector < int >(N + 1));
    while(M--) {
        int u, v, capacity, w;
        fin >> u >> v >> capacity >> w;
        Graph[u].emplace_back(v);
        Graph[v].emplace_back(u);
        Capacity[u][v] = capacity;
        Cost[u][v] = w;
        Cost[v][u] = -w;
    }
    fout << min_cost_flow();
}