hdu 1565 方格取数（神奇的最小割，模板题）

题目：http://acm.split.hdu.edu.cn/showproblem.php?pid=1565

思路：

最大流最小割：最大和=所有数之和—最小割（网络流）

建图：奇数点连源点，然后以inf为容量连周围偶数点，偶数点连汇点。

#include<cstdio>#include<cstring>#include<algorithm>#include<vector>#include<queue>#include<iostream>#define mem(a,x) memset(a,x,sizeof(a))using namespace std;const int inf =  0x3f3f3f3f;const int maxn = 50 + 5;const int maxs = maxn*maxn + 5;int n,m,source,sink;int mp[maxn][maxn];struct Edge{    int from,to,cap,flow;};struct Dicnic{    int n,m,s,t;    int d[maxs],cur[maxs],vis[maxs];    vector<int>G[maxs];    vector<Edge>edges;    void AddEdge(int from,int to,int cap){        Edge e1 = {from,to,cap,0};        Edge e2 = {to,from,0,0};        edges.push_back(e1);        edges.push_back(e2);        int m = edges.size();        G[from].push_back(m-2);        G[to].push_back(m-1);    }    void init(int n){        this -> n = n;        for(int i = 0;i<=n;i++){            G[i].clear();        }        edges.clear();    }    bool BFS(){     //就是用BFS分了个层而已，一看代码便知        mem(vis,0);        queue<int>Q;        Q.push(s);        vis[s] = 1;        d[s] = 0;        //mem(d,0);        while(!Q.empty()){            int x = Q.front();      Q.pop();            for(int i=0;i<G[x].size();i++){                Edge &e = edges[G[x][i]];                if(!vis[e.to]&&e.cap>e.flow){                    vis[e.to] = 1;                    d[e.to] = d[x] + 1;                    Q.push(e.to);                }            }        }        return vis[t];    }    int DFS(int x ,int a){  //就是顺着层次dfs而已        if(x==t||a==0)      return a;        int flow = 0 , f;        for(int &i = cur[x];i<G[x].size();i++){            Edge &e = edges[G[x][i]];            if(d[e.to]==d[x]+1&&(f = DFS(e.to,min(a,e.cap-e.flow)))>0){  //a是路径上的最小残存容量                e.flow += f;                edges[G[x][i]^1].flow -= f;  //反向边                flow += f;                a -= f;                if(a==0) break;   //a等于0及时退出，再从s那找一遍，因为从哪断的不知道            }        }        return flow;    }    int MaxFlow(int s,int t){        this -> s = s;      this ->t  = t;        int flow = 0;        while(BFS()){            mem(cur,0);            flow += DFS(s,inf);        }        return flow;    }}g;int drx[] = {0,1,0,-1};int dry[] = {1,0,-1,0};bool valid(int x,int y){    if(x<1||x>n||y<1||y>m)        return false;    else        return true;}int id(int x,int y){    return (x-1)*m + y;}int main(){    while(scanf("%d",&n)!=EOF){        m=n;        g.init(n*m + 1);        for(int i=1;i<=n;i++)            for(int j=1;j<=m;j++)                scanf("%d",&mp[i][j]);        source = 0 ; sink = n * m + 1;        int sum=0;        for(int i=1;i<=n;i++)            for(int j=1;j<=m;j++){                if((i+j)%2==1){                    g.AddEdge(source,id(i,j),mp[i][j]);                    for(int k = 0;k<4;k++){                        int nx = i + drx[k];                        int ny = j + dry[k];                        if(valid(nx,ny))                            g.AddEdge(id(i,j),id(nx,ny),inf);                    }                }                else{                    g.AddEdge(id(i,j),sink,mp[i][j]);                }                sum+=mp[i][j];            }        int ans = g.MaxFlow(source,sink);        printf("%d\n",sum-ans);    }    return 0;}/*60 31 1231 12 1 2113 135 1321 1 3 21132 132 5 15 3 1213 453 321 45 0 035 534 4 531 124 1213 135 1321 1 3 21*/