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| #include <bits/stdc++.h>
#pragma optimization_level 3
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")
#define MAX_N 2020
#define ll long long
#define pll pair<ll, ll>
#define vpll vector<pll>
#define vll vector<ll>
#define vs vector<string>
#define vvll vector<vll>
#define all(a) begin(a), end(a)
using namespace std;
bool vis[401][401];
ll dy[4]{-1,0,1,0}, dx[4]{0,1,0,-1};
string dir = "NESW";
void merge(vvll& adj, ll fy, ll fx, ll ty, ll tx, ll d) {
vpll f{{fy*2, fx*2},{fy*2, fx*2 + 1}, {fy*2 + 1, fx*2 + 1}, {fy*2 + 1, fx*2}};
vpll t{{ty*2, tx*2},{ty*2, tx*2 + 1}, {ty*2 + 1, tx*2 + 1}, {ty*2 + 1, tx*2}};
auto merge = [&] (ll a, ll b) {
pll from = f[a], to = t[b];
adj[from.first][from.second] = a;
adj[to.first][to.second] = b;
};
merge(d, (d + 2) % 4);
}
void dfs(vvll& adj, ll y, ll x) {
vis[y][x] = true;
for(ll i = 0; i < 4; i++) {
ll ny = y + dy[i], nx = x + dx[i];
if(0 <= ny*2 and ny*2 < adj.size() and 0 <= nx*2 and nx*2 < adj[ny].size() and adj[ny*2][nx*2] != -1 and !vis[ny][nx]) {
merge(adj, y, x, ny, nx, i);
dfs(adj, ny, nx);
}
}
}
string solve(vs& grid, ll& n, ll& m) {
memset(vis,0,sizeof(vis));
vvll adj(n*2, vll(m*2,0));
ll req = n*2 * m*2;
for(ll i = 0; i < n; i++)
for(ll j = 0; j < m; j++)
if(grid[i][j] == '#') {
adj[i * 2][j * 2] = adj[i * 2 + 1][j * 2] = adj[i * 2][j * 2 + 1] = adj[i * 2 + 1][j * 2 + 1] = -1;
req -= 4;
} else {
adj[i * 2][j * 2] = 1;
adj[i * 2][j * 2 + 1] = 2;
adj[i * 2 + 1][j * 2 + 1] = 3;
adj[i * 2 + 1][j * 2] = 0;
}
string res = "";
dfs(adj, 0, 0);
ll y = 0, x = 0;
do {
res.push_back(dir[adj[y][x]]);
ll ny = y + dy[adj[y][x]], nx = x + dx[adj[y][x]];
y = ny, x = nx;
}while(y | x);
return res.length() == req ? res : "IMPOSSIBLE";
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
ll tc, n, m;
cin>>tc;
for(ll i = 1; i <= tc; i++) {
cin>>n>>m;
vs grid(n);
for(ll j = 0; j < n; j++) cin>>grid[j];
cout<<"Case #"<<i<<": "<<solve(grid, n, m)<<'\n';
}
return 0;
}
|
