SPOJ 962 Intergalactic Map

SPOJ_962

    为了保证每个点只经过一次可以将点i拆成i和i'，同时连一条i->i'的容量为1的边。

    在添加边x<->y的时候，连y'->x和x'->y，最后将S连2'，1'和3'连T，看最大流是否为2即可。

#include<stdio.h>
#include<string.h>
#include<algorithm>
#define MAXD 60222
#define MAXM 460222
#define INF 0x3f3f3f3f
int N, M, first[MAXD], e, next[MAXM], v[MAXM], flow[MAXM];
int S, T, d[MAXD], q[MAXD], work[MAXD];
void add(int x, int y, int z)
{
    v[e] = y, flow[e] = z;
    next[e] = first[x], first[x] = e ++;    
}
void init()
{
    int i, x, y;
    scanf("%d%d", &N, &M);
    S = 0, T = 2 * N + 1;
    memset(first, -1, sizeof(first[0]) * (T + 1));
    e = 0;
    for(i = 0; i < M; i ++)
    {
        scanf("%d%d", &x, &y);
        if(x > N || y > N) continue;
        add(N + x, y, 1), add(y, N + x, 0);
        add(N + y, x, 1), add(x, N + y, 0);
    }
    for(i = 1; i <= N; i ++)
        add(i, N + i, 1), add(N + i, i, 0);
    add(S, N + 2, 2), add(N + 2, S, 0);
    add(N + 1, T, 1), add(T, N + 1, 0), add(N + 3, T, 1), add(T, N + 3, 0);
}
int bfs()
{
    int i, j, rear = 0;
    memset(d, -1, sizeof(d[0]) * (T + 1));    
    d[S] = 0, q[rear ++] = S;
    for(i = 0; i < rear; i ++)
    {
        for(j = first[q[i]]; j != -1; j = next[j])
            if(flow[j] && d[v[j]] == -1)
            {
                d[v[j]] = d[q[i]] + 1, q[rear ++] = v[j];
                if(v[j] == T)
                    return 1;    
            }
    }
    return 0;
}
int dfs(int cur, int a)
{
    if(cur == T)
        return a;
    int t;
    for(int &i = work[cur]; i != -1; i = next[i])
        if(flow[i] && d[v[i]] == d[cur] + 1)
            if(t = dfs(v[i], std::min(a, flow[i])))
            {
                flow[i] -= t, flow[i ^ 1] += t;
                return t;    
            }
    return 0;
}
int dinic()
{
    int ans = 0, t;
    while(bfs())
    {
        memcpy(work, first, sizeof(first[0]) * (T + 1));
        while(t = dfs(S, INF))
            ans += t;
    }
    return ans;
}
void solve()
{
    if(N < 3 || dinic() != 2)
        printf("NO\n");
    else
        printf("YES\n");
}
int main()
{
    int t;
    scanf("%d", &t);
    while(t --)
    {
        init();
        solve();    
    }
    return 0;    
}