BZOJ 1001：[BeiJing2006]狼抓兔子（最小割）

http://www.lydsy.com/JudgeOnline/problem.php?id=1001

题意：中文。

思路：很明显是最小割，转化为最大流做。一开始看那么多点，但还是试了一下，居然过了。迷。

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <queue>
using namespace std;
#define N 1000010
#define INF 0x3f3f3f3f
struct Edge {
    int v, cap, nxt;
} edge[N*6];
int head[N], cur[N], pre[N], tot, gap[N], dis[N], S, T;

void Add(int u, int v, int cap) {
    edge[tot] = (Edge) {v, cap, head[u]}; head[u] = tot++;
    edge[tot] = (Edge) {u, cap, head[v]}; head[v] = tot++;
}

int BFS() {
    memset(gap, 0, sizeof(gap));
    memset(dis, INF, sizeof(dis));
    queue<int> que;
    que.push(T);
    dis[T] = 0; gap[0] = 1;
    while(!que.empty()) {
        int u = que.front(); que.pop();
        for(int i = head[u]; ~i; i = edge[i].nxt) {
            int v = edge[i].v;
            if(INF != dis[v]) continue;
            dis[v] = dis[u] + 1;
            gap[dis[v]]++;
            que.push(v);
        }
    }
}

int ISAP(int n) {
    BFS();
    memcpy(cur, head, sizeof(cur));
    int flow, ans = 0, u = pre[S] = S, i, index;
    while(dis[S] < n) {
        if(u == T) {
            flow = INF;
            for(i = S; i != T; i = edge[cur[i]].v)
                if(flow > edge[cur[i]].cap) flow = edge[cur[i]].cap, index = i;
            for(i = S; i != T; i = edge[cur[i]].v)
                edge[cur[i]].cap -= flow, edge[cur[i]^1].cap += flow;
            ans += flow; u = index;
        }
        for(i = cur[u]; ~i; i = edge[i].nxt) if(edge[i].cap > 0 && dis[edge[i].v] == dis[u] - 1) break;
        if(~i) {
            cur[u] = i; pre[edge[i].v] = u; u = edge[i].v;
        } else {
            if(--gap[dis[u]] == 0) break;
            int md = n + 1;
            for(i = head[u]; ~i; i = edge[i].nxt)
                if(edge[i].cap > 0 && dis[edge[i].v] < md) md = dis[edge[i].v], cur[u] = i;
            gap[dis[u] = md + 1]++;
            u = pre[u];
        }
    }
    return ans;
}

int main() {
    int n, m, w;
    scanf("%d%d", &n, &m);
    memset(head, -1, sizeof(head));
    tot = 0;
    for(int i = 0; i < n; i++)
        for(int j = 2; j <= m; j++)
            scanf("%d", &w), Add(j - 1 + i * m, j + i * m, w);
    for(int i = 1; i < n; i++)
        for(int j = 1; j <= m; j++)
            scanf("%d", &w), Add(j + (i - 1) * m, j + i * m, w);
    for(int i = 1; i < n; i++)
        for(int j = 2; j <= m; j++)
            scanf("%d", &w), Add(j - 1 + (i - 1) * m, j + i * m, w);
    S = 1, T = n * m;
    printf("%d
", ISAP(T + 1));
    return 0;
}