LibreOJ 6007:「网络流 24 题」方格取数题解

解法

这道题可以看作是网络流的一个模型了。我们把棋盘染色成红色和黑色。然后,源点连红色点,最大流限制为红点的点权;黑点全部连到汇点,最大流限制为黑点的点权。答案为点权总和减去最大流。

我们可以尝试理解一下:对于\(m = 1, n = 2\)的情况:

\[ [ x, y ] \]

那么假设\(x\)为黑点,\(y\)为红点,在网络上是这样的:

我们会发现最大流只会留下较小的一项。这样的话,我们可以感性推广到任何情况,大难点权和减掉最小割就行了。

代码

// LOJ6007.cpp
#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 10003, INF = 0x3f3f3f3f;
const int vertical[4] = {0, 1, -1, 0}, hori[4] = {1, 0, 0, -1};
int n, m, sum, head[MAX_N], current, dep[MAX_N], s, t, cur[MAX_N];
int arr[50][50], color, colors[50][50];
struct edge
{
    int to, nxt, weight;
} edges[80010];
void addpath(int src, int dst, int weight)
{
    edges[current].to = dst, edges[current].nxt = head[src];
    edges[current].weight = weight, head[src] = current++;
}
void add(int src, int dst, int weight)
{
    addpath(src, dst, weight);
    addpath(dst, src, 0);
}
bool bfs()
{
    memset(dep, 0, sizeof(dep));
    queue<int> q;
    q.push(s), dep[s] = 1;
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        for (int i = head[u]; i != -1; i = edges[i].nxt)
            if (edges[i].weight > 0 && dep[edges[i].to] == 0)
            {
                dep[edges[i].to] = dep[u] + 1;
                q.push(edges[i].to);
            }
    }
    return dep[t] != 0;
}
int dfs(int u, int flow)
{
    if (flow == 0 || u == t)
        return flow;
    for (int &i = cur[u]; i != -1; i = edges[i].nxt)
        if (dep[edges[i].to] == dep[u] + 1 && edges[i].weight > 0)
            if (int di = dfs(edges[i].to, min(flow, edges[i].weight)))
            {
                edges[i].weight -= di, edges[i ^ 1].weight += di;
                return di;
            }
    return 0;
}
int dinic()
{
    int ans = 0;
    while (bfs())
    {
        for (int i = 0; i <= t; i++)
            cur[i] = head[i];
        while (int di = dfs(s, INF))
            ans += di;
    }
    return ans;
}
int getPos(int x, int y) { return (x - 1) * m + y; }
int main()
{
    memset(head, -1, sizeof(head));
    scanf("%d%d", &n, &m);
    s = 0, t = n * m + 1;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            scanf("%d", &arr[i][j]), colors[i][j] = (i + j) & 1, sum += arr[i][j];
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            if (colors[i][j] == 1)
                add(getPos(i, j), t, arr[i][j]);
            else
                add(s, getPos(i, j), arr[i][j]);
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            if (colors[i][j] == 0)
                for (int dir = 0; dir < 4; dir++)
                {
                    int dstX = i + vertical[dir], dstY = j + hori[dir];
                    if (dstX > 0 && dstX <= n && dstY > 0 && dstY <= m)
                        add(getPos(i, j), getPos(dstX, dstY), INF);
                }
    int answer = sum - dinic();
    printf("%d", answer);
    return 0;
}

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Kal0rona

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