【USACO 5.4.3】TeleCowmunication

/*
TASK:telecow
LANG:C++
*/
#include <cstdio>
#include <cstring>
#include <queue>

using namespace std;

const int INF = 0x7fffffff;

struct Edge
{
    int c, f;
    bool canget;
    
    Edge() {
        canget = false;
    }
    
    Edge(int cap, int flow) : c(cap), f(flow) {
        canget = true;
    }
    
}net[205][205];

int n, m, c1, c2, netflow, d[205], side[605][2];

bool BFS()
{
    memset(d, 0, sizeof(d));
    d[2 * c1] = 1;
    queue<int> Q;
    Q.push(2 * c1);
    while (!Q.empty())
    {
        int x = Q.front();
        Q.pop();
        for (int i = 1; i <= 2 * n; ++i)
            if (d[i] == 0 && net[x][i].canget && net[x][i].f < net[x][i].c)
            {
                d[i] = d[x] + 1;
                Q.push(i);
            }
    }
    return d[2 * c2 - 1];
}

int DFS(int x, int a)
{
    if (x == 2 * c2 - 1 || a == 0) return a;
    int flow = 0;
    for (int i = 1; i <= 2 * n; ++i)
    {
        int fw;
        if (d[x] + 1 == d[i] && (fw = DFS(i, min(a, net[x][i].c - net[x][i].f))) > 0)
        {
            net[x][i].f += fw;
            net[i][x].f -= fw;
            flow += fw;
            a -= fw;
        }
        if (a == 0) break;
    }
    return flow;
}

int main()
{
    freopen("telecow.in", "r", stdin);
    freopen("telecow.out", "w", stdout);
    scanf("%d%d%d%d", &n, &m, &c1, &c2);
    for (int i = 1; i <= n; ++i)
    {
        net[2 * i - 1][2 * i] = Edge(1, 0);
        net[2 * i][2 * i - 1] = Edge(0, -1);
    }
    for (int i = 0; i < m; ++i)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        side[i][0] = a;
        side[i][1] = b;
        net[2 * a][2 * b - 1] = Edge(INF, 0);
        net[2 * b - 1][2 * a] = Edge(0, 0);
        net[2 * b][2 * a - 1] = Edge(INF, 0);
        net[2 * a - 1][2 * b] = Edge(0, 0);
    }
    netflow = 0;
    net[2 * c1 - 1][2 * c1].canget = net[2 * c1][2 * c1 - 1].canget = false;
    net[2 * c2 - 1][2 * c2].canget = net[2 * c2][2 * c2 - 1].canget = false;
    while (BFS())
    {
        netflow += DFS(2 * c1, INF);
    }
    printf("%d\n", netflow);
    for (int k = 1; k <= n; ++k)
    {
        if (k == c1 || k == c2) continue;
        for (int i = 1; i <= n; ++i)
        if (net[2 * i - 1][2 * i].canget){
            net[2 * i - 1][2 * i] = Edge(1, 0);
            net[2 * i][2 * i - 1] = Edge(0, -1);
        }
        net[2 * k - 1][2 * k].canget = net[2 * k][2 * k - 1].canget = false;
        for (int i = 0; i < m; ++i)
        {
            int a = side[i][0], b = side[i][1];
            net[2 * a][2 * b - 1] = Edge(INF, 0);
            net[2 * b - 1][2 * a] = Edge(0, 0);
            net[2 * b][2 * a - 1] = Edge(INF, 0);
            net[2 * a - 1][2 * b] = Edge(0, 0);
        }
        int sumflow = 0;
        while (BFS())
        {
            sumflow += DFS(2 * c1, INF);
        }
        if (sumflow < netflow)
        {
            netflow = sumflow;
            printf("%d", k);
            if (netflow == 0) break;
            else printf(" ");
        }
        else
        {
            net[2 * k - 1][2 * k].canget = net[2 * k][2 * k - 1].canget = true;
        }
    }
    printf("\n");
    return 0;
}