more cf

cses/introductory-problems/grid-paths.cc

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+#include <bits/stdc++.h>  // {{{
+
+// https://codeforces.com/blog/entry/96344
+
+#pragma GCC optimize("O2,unroll-loops")
+#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
+
+using namespace std;
+
+template <typename T>
+constexpr T MIN = std::numeric_limits<T>::min();
+
+template <typename T>
+constexpr T MAX = std::numeric_limits<T>::max();
+
+template <typename T>
+[[nodiscard]] static T sc(auto&& x) {
+  return static_cast<T>(x);
+}
+
+template <typename T>
+[[nodiscard]] static T sz(auto&& x) {
+  return static_cast<T>(x.size());
+}
+
+using ll = long long;
+using ld = long double;
+template <typename T>
+using vec = std::vector<T>;
+template <typename T, size_t N>
+using arr = std::array<T, N>;
+
+#define ff first
+#define ss second
+#define eb emplace_back
+#define pb push_back
+#define all(x) (x).begin(), (x).end()
+#define rall(x) (x).rbegin(), (x).rend()
+//  }}}
+
+bitset<7 * 7 + 1> seen;
+
+vec<pair<int, int>> dirs;
+unordered_map<char, pair<int, int>> dir_map{
+    {'U', {-1, 0}}, {'D', {1, 0}}, {'L', {0, -1}}, {'R', {0, 1}}};
+
+ll ans = 0;
+
+string s;
+
+void dfs(int r, int c, int i) {
+  if (min(r, c) < 0 || max(r, c) > 6 || seen[r * 7 + c]) {
+    return;
+  }
+  if (r == 6 && c == 0) {
+    ans += i == 48;
+    return;
+  }
+  seen[r * 7 + c] = true;
+  bool up = (r > 0 && !seen[(r - 1) * 7 + c]);
+  bool down = (r < 6 && !seen[(r + 1) * 7 + c]);
+  bool left = (c > 0 && !seen[r * 7 + (c - 1)]);
+  bool right = (c < 6 && !seen[r * 7 + (c + 1)]);
+
+  if ((up && down && !left && !right) || (!up && !down && left && right)) {
+    seen[r * 7 + c] = false;
+    return;
+  }
+  int open_count = up + down + left + right;
+    if (open_count == 1) {
+        if (up) dfs(r - 1, c, i + 1);
+        else if (down) dfs(r + 1, c, i + 1);
+        else if (left) dfs(r, c - 1, i + 1);
+        else if (right) dfs(r, c + 1, i + 1);
+        
+        seen[r * 7 + c] = false;
+        return;
+    }
+  if (s[i] == '?') {
+    for (auto& [dr, dc] : dirs) {
+      dfs(r + dr, c + dc, i + 1);
+    }
+  } else {
+    dfs(r + dir_map[s[i]].ff, c + dir_map[s[i]].ss, i + 1);
+  }
+  seen[r * 7 + c] = false;
+}
+
+void solve() {
+  dirs.eb(1, 0);
+  dirs.eb(0, 1);
+  dirs.eb(-1, 0);
+  dirs.eb(0, -1);
+  cin >> s;
+
+  dfs(0, 0, 0);
+
+  cout << ans << endl;
+}
+
+int main() {  // {{{
+  cin.tie(nullptr)->sync_with_stdio(false);
+
+  solve();
+
+  return 0;
+}
+// }}}