## Wednesday, March 11, 2015

### UVa 563 - Crimewave

Problem Statement:
UVa 563 - Crimewave

Solution:
At first, the problem seems to be of an undirected graph maximum flow, which can be tedious to do. Furthermore, each vertices (i.e. crossings) on the map has a capacity of 1. We need to transform this representation into a more friendly one, and as it turns out, we can transform this problem into our old familiar directed graph maximum flow.

The main idea of the solution is to represent each "crossing" C as two nodes C(in) and C(out), where a directed edge of capacity 1 from C(in) is drawn to C(out). Furthermore, for each neighbours of C, namely K, L, M, and N, we connect C(out) to each K(in), L(in), M(in), and N(in), all having capacity 1. Finally, we connect source S to all banks B(in) with capacity 1, and connect all crossings on the edges E(out) to our sink T with capacity 1. The rest is simply a careful implementation of Edmonds Karp algorithm. While the running time is bounded at O(VE^2), this bound is actually way too large. This is because since there are at most 50 * 50 * 2 = 200 escape crossings on the edges of the map, the running of Edmonds Karp is bounded to at most 200 iterations of BFS. This leads to a very manageable running time complexity.

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <utility>
#include <vector>
#include <queue>
using namespace std;

struct edge {
int u;
int v;
int cap;
int flow;
edge(int u, int v, int cap, int flow) {
this -> u = u;
this -> v = v;
this -> cap = cap;
this -> flow = flow;
}
};

int INF = 1234567;
vector<edge> edges;
int vis[6000];
int par[6000];
int M, N, B;
int S, T;
int maxflow;

int augment_path(int v, int mf) {
if(v == S) {
return mf;
}
edge &e = edges[par[v]];
int u;
bool isBackEdge = (e.v != v);
if(isBackEdge) {
u = e.v;
mf = min(e.flow, mf);
} else {
u = e.u;
mf = min(e.cap - e.flow, mf);
}

mf = augment_path(u, mf);
if(isBackEdge) {
e.flow -= mf;
} else {
e.flow += mf;
}
return mf;
}

void edmondskarp() {
while(1) {
bool augmented = false;
queue<int> q;
q.push(S);
vis[i] = 0;
}
vis[S] = 1;
while(!q.empty()) {
int u = q.front();
q.pop();
if(vis[v])continue;
if(e.u == u && e.v == v) {
if(e.cap - e.flow > 0) {
vis[v] = 1;
q.push(v);
}
} else {
if(e.flow > 0) {
vis[v] = 1;
q.push(v);
}
}
if(v == T) {
augmented = true;
maxflow += augment_path(T, INF);
break;
}
}
}
if(augmented) break;
}
if(!augmented) break;
}
}

int getIndexIn(int x, int y) {
return (x-1) * N + (y-1);
}

int getIndexOut(int x, int y) {
return getIndexIn(x,y) + M*N;
}

void makeEdge(int u, int v) {
edges.push_back(edge(u, v, 1, 0));
}

int main(){
int tc;
scanf("%d",&tc);
int x, y;
while(tc--){
edges.clear();
scanf("%d%d%d",&M,&N,&B);
S = 2*M*N;
T = 2*M*N+1;
for(int i=1;i<=M;++i){
for(int j=1;j<=N;++j){
makeEdge(getIndexIn(i,j), getIndexOut(i,j));
if(i-1>0) {
makeEdge(getIndexOut(i,j), getIndexIn(i-1,j));
}
if(j-1>0) {
makeEdge(getIndexOut(i,j), getIndexIn(i, j-1));
}
if(i+1 <= M) {
makeEdge(getIndexOut(i,j), getIndexIn(i+1, j));
}
if(j+1 <= N) {
makeEdge(getIndexOut(i,j), getIndexIn(i, j+1));
}
if(i-1 == 0 || j-1 == 0 || i+1 >M || j+1 > N) {
makeEdge(getIndexOut(i,j), T);
}
}
}
bool ok = true;
for(int i=0;i<B;++i){
scanf("%d%d",&x,&y);
makeEdge(S, getIndexIn(x,y));
}
if(!ok)continue;
maxflow = 0;
edmondskarp();
//cout << maxflow << endl;
if(maxflow == B) printf("possible\n");
else printf("not possible\n");
}
return 0;
}