UVA, 11380Down Went the Titanic (max. Flow, split)

Maximum Flow, split

Test instructions: @ can walk infinitely, * person's initial position, others cannot walk. ~ Water, can't go, ... Can only walk once, #终点, can accommodate the number of p; Ask at most how many people were rescued (arrived #)

This question is the biggest flow problem, mainly to build the diagram. How to build a diagram? Mainly with a split point, a point is split into two points, point (I,J) can be expressed as: Front point (i-1) *a+j, after the point (i-1) *a+j+m (m for a larger number, to ensure that M is greater than or equal to s*a on the line), and then connect the front and back points, the direction is from point to point, @ and Their front and back-point weights are set infinitely large.

Adjacent points, the graph is non-direction, with the point of the back point of the adjacent points to connect the front point, if the adjacent point is @ or #, set the weight of the edge infinity.

Connect all the front points with a super source point, connect all the # points with a super endpoint, the weights for all sides are p, the other didn't see? edge weights Default to 1, and the graph is built. Then use the maximum flow algorithm that you are familiar with.

Ford-fulkerson Maximum Flow Algorithm code reference: http://blog.csdn.net/itaskyou/article/details/51331344

#include <iostream> #include <vector> #include <cstring> #include <map> #include <string>    using namespace std; #define N 2005#define INF 1000000struct edge{int to,cap,rev;        Edge (int a,int b,int c) {to=a;        Cap=b;    Rev=c;  }};vector<edge>v[n];void add_edge (int from,int to,int cap); int dfs (int a,int t,int f); int max_flow (int s,int t); int    Used[n];int q[4][2]= {1,0,0,1,-1,0,0,-1};string s[n];int main () {int x,y,p;         while (cin>>x>>y>>p) {for (int i=0;i<n;i++) {v[i].clear ();//Initialize;}        for (int i=0;i<x;i++) {cin>>s[i]; } for (int i=0;i<x;i++)//split {for (int j=0;j<y;j++) {if (s[i][j]== '                    # ') {Add_edge (i*y+j,i*y+j+x*y,inf);                Add_edge (I*Y+J+X*Y,X*Y*2+1,P);             } else if (s[i][j]== ' @ ') {       Add_edge (I*y+j,i*y+j+x*y,inf);                    } else if (s[i][j]== ' * ') {Add_edge (i*y+j,i*y+j+x*y,1);                Add_edge (x*y*2,i*y+j,1);                 } else if (s[i][j]== '. ')                 {Add_edge (i*y+j,i*y+j+x*y,1);                     } for (int k=0;k<4;k++) {int k1=i+q[k][0];                     int k2=j+q[k][1]; if (k1>=0&&k1<x&&k2>=0&&k2<y) {if (s[k1][k2 ]=='@'||                         s[k1][k2]== ' # ') Add_edge (I*y+j+x*y,k1*y+k2,inf);                     else Add_edge (i*y+j+x*y,k1*y+k2,1);        }}}} int Ans=max_flow (x*y*2,x*y*2+1);    cout<<ans<<endl; } return 0;} void Add_edge (int from,int to,int cap) {V[from].push_back (Edge (To,cap,v[to].size ())); V[to].push_back (Edge (From,0,v[from].size ()-1));}    int dfs (int a,int t,int f) {if (a==t) return F;    Used[a]=1;        for (int i=0;i<v[a].size (); i++) {Edge &e=v[a][i];            if (!used[e.to]&&e.cap>0) {int D=dfs (e.to,t,min (F,e.cap));                if (d>0) {e.cap-=d;                V[e.to][e.rev].cap+=d;            return D; }}} return 0;}    int max_flow (int s,int t) {int flow=0;        while (1) {memset (used,0,sizeof (used));        int F=dfs (s,t,inf);        if (f==0) return flow;    Flow+=f; }}

UVA, 11380Down Went the Titanic (max. Flow, split)