Problem p00236 (C++) — buggy submission, verdict: Memory Limit Exceeded

#include <algorithm>
#include <bitset>
#include <iostream>
#include <set>
#include <vector>
using namespace std;

const int N = 49;

int w, h;
vector<vector<int>> field;
int size;
vector<vector<int>> vertex;
vector<vector<int>> edge;
bitset<N> visited;
set<unsigned long long> memo;

void make_graph() {
  enum { Y, X };
  const int NEXT[4][2] = {{0, 1}, {1, 0}, {-1, 0}, {0, -1}};
  //     const int NEXT[4][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
  const int NIL = -1;
  size = 0;
  vertex = vector<vector<int>>(h, vector<int>(w, NIL));
  for (int y = 0; y < h; ++y)
    for (int x = 0; x < w; ++x)
      if (!field[y][x])
        vertex[y][x] = size++;
  edge = vector<vector<int>>(h * w, vector<int>());
  for (int y = 0; y < h; ++y)
    for (int x = 0; x < w; ++x) {
      if (vertex[y][x] == NIL)
        continue;
      for (const auto &d : NEXT) {
        int ny = y + d[Y];
        int nx = x + d[X];
        if (ny < 0 || h <= ny || nx < 0 || w <= nx)
          continue;
        if (vertex[ny][nx] == NIL)
          continue;
        edge[vertex[y][x]].push_back(vertex[ny][nx]);
      }
    }
};

void reachability(const int &from, bitset<N> &visited) {
  visited.set(from);
  for (const auto &to : edge[from])
    if (!visited[to])
      reachability(to, visited);
}
bool cut_vertex(const int &from) {
  auto dup = visited;
  dup.set(from);
  for (const auto &to : edge[from]) {
    if (dup[to])
      continue;
    reachability(to, dup);
    return dup.count() != size;
  }
  return dup.count() != size;
}

bool dfs(const int &start, int from) {
  //     cout<<visited<<" "<<from<<endl;
  unsigned long long m = visited.to_ullong() * N + from;
  if (memo.count(m))
    return false;
  memo.insert(m);

  // cut vertexはダメ
  if (cut_vertex(from))
    return false;

  // 現在地以外で次数1の頂点があるとダメ
  for (int u = 0; u < size; ++u) {
    if (u == from)
      continue;
    if (visited[u])
      continue;
    int degree = 0;
    for (const auto &v : edge[u])
      if (v == start || !visited[v])
        ++degree;
    if (degree < 2)
      return false;
  }

  visited.set(from);
  for (const auto &to : edge[from]) {
    if (to == start && visited.count() == size)
      return true;
    if (visited[to])
      continue;
    if (dfs(start, to))
      return true;
  }
  visited.reset(from);
  return false;
}

bool solve() {
  visited = bitset<N>(0);
  make_graph();
  memo.clear();
  for (int y = 0; y < h; ++y)
    for (int x = 0; x < w; ++x)
      if (!field[y][x])
        return dfs(vertex[y][x], vertex[y][x]);
  //     for(int y = 3; y < h; ++y) for(int x = 0; x < w; ++x) if(!field[y][x])
  //     return dfs(vertex[y][x], vertex[y][x]); for(int y = 0; y < 3; ++y)
  //     for(int x = 0; x < w; ++x) if(!field[y][x]) return dfs(vertex[y][x],
  //     vertex[y][x]);
  return false;
}

int main() {
  while (cin >> w >> h, w | h) {
    field = vector<vector<int>>(h, vector<int>(w));
    for (auto &line : field)
      for (auto &v : line)
        cin >> v;
    cout << (solve() ? "Yes" : "No") << endl;
  }
}

Error: MLE

Fixed submission:

#include <algorithm>
#include <bitset>
#include <iostream>
#include <set>
#include <vector>
using namespace std;

const int N = 49;

int w, h;
vector<vector<int>> field;
int size;
vector<vector<int>> vertex;
vector<vector<int>> edge;
bitset<N> visited;
set<unsigned long long> memo;

void make_graph() {
  enum { Y, X };
  const int NEXT[4][2] = {{0, 1}, {1, 0}, {-1, 0}, {0, -1}};
  //     const int NEXT[4][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
  const int NIL = -1;
  size = 0;
  vertex = vector<vector<int>>(h, vector<int>(w, NIL));
  for (int y = 0; y < h; ++y)
    for (int x = 0; x < w; ++x)
      if (!field[y][x])
        vertex[y][x] = size++;
  edge = vector<vector<int>>(h * w, vector<int>());
  for (int y = 0; y < h; ++y)
    for (int x = 0; x < w; ++x) {
      if (vertex[y][x] == NIL)
        continue;
      for (const auto &d : NEXT) {
        int ny = y + d[Y];
        int nx = x + d[X];
        if (ny < 0 || h <= ny || nx < 0 || w <= nx)
          continue;
        if (vertex[ny][nx] == NIL)
          continue;
        edge[vertex[y][x]].push_back(vertex[ny][nx]);
      }
    }
};

void reachability(const int &from, bitset<N> &visited) {
  visited.set(from);
  for (const auto &to : edge[from])
    if (!visited[to])
      reachability(to, visited);
}
bool cut_vertex(const int &from) {
  auto dup = visited;
  dup.set(from);
  for (const auto &to : edge[from]) {
    if (dup[to])
      continue;
    reachability(to, dup);
    return dup.count() != size;
  }
  return dup.count() != size;
}

bool dfs(const int &start, int from) {
  //     cout<<visited<<" "<<from<<endl;
  unsigned long long m = visited.to_ullong() * N + from;
  if (memo.count(m))
    return false;
  memo.insert(m);

  // startの四方が囲まれていたらダメ
  bool deadend = true;
  for (const auto &v : edge[start])
    deadend &= visited[v];
  if (deadend)
    return false;

  // 現在地以外で次数1の頂点があるとダメ
  for (int u = 0; u < size; ++u) {
    if (u == from)
      continue;
    if (visited[u])
      continue;
    int degree = 0;
    for (const auto &v : edge[u])
      if (v == start || !visited[v])
        ++degree;
    if (degree < 2)
      return false;
  }

  visited.set(from);
  for (const auto &to : edge[from]) {
    if (to == start && visited.count() == size)
      return true;
    if (visited[to])
      continue;
    if (dfs(start, to))
      return true;
  }
  visited.reset(from);
  return false;
}

bool solve() {
  visited = bitset<N>(0);
  make_graph();
  memo.clear();
  for (int y = 0; y < h; ++y)
    for (int x = 0; x < w; ++x)
      if (!field[y][x])
        return dfs(vertex[y][x], vertex[y][x]);
  //     for(int y = 3; y < h; ++y) for(int x = 0; x < w; ++x) if(!field[y][x])
  //     return dfs(vertex[y][x], vertex[y][x]); for(int y = 0; y < 3; ++y)
  //     for(int x = 0; x < w; ++x) if(!field[y][x]) return dfs(vertex[y][x],
  //     vertex[y][x]);
  return false;
}

int main() {
  while (cin >> w >> h, w | h) {
    field = vector<vector<int>>(h, vector<int>(w));
    for (auto &line : field)
      for (auto &v : line)
        cin >> v;
    cout << (solve() ? "Yes" : "No") << endl;
  }
}