(Analysis by Brian Dean)

This problem is an exercise in recursive (depth first) searching to identify the connected components in a graph.

The first task is much simpler than the second. We make a graph where every cell is a node, and two adjacent cells are connected with an edge if they contain the same number. We then launch a recursive "flood fill" from every cell (skipping the ones already visited) that fans out to label every connected region while adding up its size. My code below actually does something slightly different -- but ultimately equivalent -- by building a separate graph for each distinct ID number and then recursively searching through all of these (this allows me to re-use most of the code I wrote for the first task for the second task).

For the second task, we build a graph for every pair of cows $(x,y)$ where the nodes are the regions we computed in the first task, and the edges connect adjacent regions where one is labeled by cow $x$ and the other by cow $y$. Each node in this graph is given a "size" which is the same as the corresponding region size from the first task. We then launch a recursive flood fill on each of these graphs to find whichever one has the largest region.

There are two important ideas to employ here in order to make our solutions run quickly. One is to make sure each region from the first task gets compressed to a single node in the second task. If we don't do this, then in the second task we might end up recursively scanning through the same large region over and over, wasting considerable time. The other idea is that when we do our recursive searching, we need to be sure the running time only depends on the number of edges in the graph, not the number of nodes, since each of the $(x,y)$ pairwise graphs for the second task involves a large number of nodes but likely very few edges. For example, we would not want to keep an array of "have I been there" flags for every node, which is initialized to false for each recursive search. Instead, in the solution below, we use a map data structure for this purpose, which only creates flags as they are needed, avoiding the initialization of flags for nodes that ultimately end up not being relevant.

My code is below. (it's a bit on the terse side, though, so if anyone has alternative solutions that are cleaner to look at please feel welcome to send them my way to see if they are better suited for use as a model solution).

#include <iostream>
#include <fstream>
#include <map>
#include <set>
#include <algorithm>
using namespace std;

int N, B[1001][1001], cellid[1001][1001], global_regid;

struct Graph {
map<int,set<int>> out_edges;
map<int,int> nodesize, regid;  // size of each node and region ID to which it belongs
map<int,int> regsize;          // size of each region
};

typedef pair<int,int> pii;
map<int, Graph> G1; // Graphs for all possible single IDs
map<pii, Graph> G2; // Graphs for all possible adjacent region pairs

// Return size of region reachable from nodeid
int visit(Graph &G, int nodeid, int regid)
{
if (G.regid.count(nodeid) > 0) return 0;  // already visited?  bail out
G.regid[nodeid] = regid;                  // mark this node as visited
int a = G.nodesize[nodeid];               // tally up region size
for (int nbrid : G.out_edges[nodeid])
a += visit(G, nbrid, regid);
G.regsize[regid] = a;
return a;
}

// Compute region sizes and return largest region size in graph.
// Running time only depends on # of edges, not # of nodes, so graph can be very sparse
int largest_region(Graph &G)
{
int m = 0;
for (auto &p : G.out_edges) m = max(m, visit(G, p.first, ++global_regid));
return m;
}

void add_edge(Graph &G, int node1, int node2)
{
G.out_edges[node1].insert(node2);
G.out_edges[node2].insert(node1);
G.nodesize[node1] = G.nodesize[node2] = 1;
}

// Add edge between two regions in a region pair graph
void add_G2_edge(int i1, int j1, int i2, int j2)
{
int b1 = B[i1][j1], b2 = B[i2][j2], c1 = cellid[i1][j1], c2 = cellid[i2][j2];
if (b1 > b2) { swap (b1,b2); swap(c1,c2); }
int r1 = G1[b1].regid[c1], r2 = G1[b2].regid[c2];
pii p = make_pair(b1, b2);
G2[p].nodesize[r1] = G1[b1].regsize[r1];
G2[p].nodesize[r2] = G1[b2].regsize[r2];
}

int main(void)
{
ifstream fin ("multimoo.in");
ofstream fout ("multimoo.out");

fin >> N;
for (int i=1; i<=N; i++)
for (int j=1; j<=N; j++) {
fin >> B[i][j];
cellid[i][j] = i*N+j;    // unique ID for each cell
}

// Build primary graph
for (int i=1; i<=N; i++)
for (int j=1; j<=N; j++) {
if (i<N && B[i+1][j] == B[i][j]) add_edge(G1[B[i][j]], cellid[i][j], cellid[i+1][j]);
if (j<N && B[i][j+1] == B[i][j]) add_edge(G1[B[i][j]], cellid[i][j], cellid[i][j+1]);
}

// Find largest region in primary graph

// Build secondary graphs based on regions of the primary graph that are adjacent
for (int i=1; i<=N; i++)
for (int j=1; j<=N; j++) {
if (i<N && B[i+1][j] != B[i][j]) add_G2_edge(i,j,i+1,j);
if (j<N && B[i][j+1] != B[i][j]) add_G2_edge(i,j,i,j+1);
}

// Find largest region in secondary graphs