URAL 1277 Cops and Thieves

URAL_1277

    拆点之后做最大流即可，但要注意S和F在同一个位置的情况。

#include<stdio.h>
#include<string.h>
#include<algorithm>
#define MAXD 210
#define MAXM 40410
#define INF 0x3f3f3f3f
int a[MAXD], N, M, S, F, K, first[MAXD], e, next[MAXM], v[MAXM], flow[MAXM];
int 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%d%d", &N, &M, &S, &F);
    for(i = 1; i <= N; i ++)
        scanf("%d", &a[i]);
    memset(first + 1, -1, sizeof(first[0]) * N << 1);
    e = 0;
    for(i = 0; i < M; i ++)
    {
        scanf("%d%d", &x, &y);
        add(x + N, y, INF), add(y, x + N, 0);
        add(y + N, x, INF), add(x, y + N, 0);
    }
    for(i = 1; i <= N; i ++)
        add(i, i + N, a[i]), add(i + N, i, 0);
}
int bfs()
{
    int i, j, rear = 0;
    memset(d + 1, -1, sizeof(d[0]) * N << 1);
    d[S + N] = 0, q[rear ++] = S + N;
    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] == F)
                    return 1;
            }
    //printf(">>> %d %d %d %d\n", S, first[S], flow[7], d[7]);
    return 0;
}
int dfs(int cur, int a)
{
    if(cur == F)
        return a;
    for(int &i = work[cur]; i != -1; i = next[i])
        if(flow[i] && d[v[i]] == d[cur] + 1)
            if(int 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 + 1, first + 1, sizeof(first[0]) * N << 1);
        while(t = dfs(S + N, INF))
            ans += t;
    }
    return ans;
}
void solve()
{
    printf("%s\n", dinic() <= K ? "YES" : "NO");
}
int main()
{
    while(scanf("%d", &K) == 1)
    {
        init();
        if(S == F)
            printf("NO\n");
        else
            solve();
    }
    return 0;
}