func_code_string: #include <bits/stdc++.h>   using namespace std;   template <class T>   bool chmax(T& a, const T& b) {     if (a < b) {       a = b;       return 1;     }     return 0;   }   template <class T>   bool chmin(T& a, const T& b) {     if (b < a) {       a = b;       return 1;     }     return 0;   }   template <typename T>   class Grid {     struct Pos {       int i, j;       Pos(int i, int j) : i(i), j(j) {}     };     int H, W;     vector<vector<T>> G;     vector<int> di8 = {1, 1, 0, -1, -1, -1, 0, 1},                 dj8 = {0, 1, 1, 1, 0, -1, -1, -1};     vector<int> di4 = {1, 0, -1, 0}, dj4 = {0, 1, 0, -1};     vector<Pos> neighbor(int i, int j, const vector<int>& dis,                          const vector<int>& djs) const {       vector<Pos> res;       for (int k = 0; k < dis.size(); ++k) {         if (in(i + dis[k], j + djs[k])) res.emplace_back(i + dis[k], j + djs[k]);       }       return res;     }       public:     static bool in(int H, int W, int i, int j) {       return 0 <= i && i < H && 0 <= j && j < W;     }     Grid(int H, int W) : H(H), W(W) { G.resize(H, vector<T>(W)); }     Grid(int H, int W, T zero) : H(H), W(W) { G.resize(H, vector<T>(W, zero)); }     vector<T>& operator[](int i) { return G[i]; }     bool in(int i, int j) const { return (0 <= i && i < H && 0 <= j && j < W); }     vector<Pos> neighbor8(int i, int j) const { return neighbor(i, j, di8, dj8); }     vector<Pos> neighbor4(int i, int j) const { return neighbor(i, j, di4, dj4); }     int to1DIndex(int i, int j) { return i * W + j; }     Pos to2DIndex(int i) { return Pos(i / W, i % W); }     void scanFrom(istream& strm) {       for (int i = 0; i < (int)(H); ++i)         for (int j = 0; j < (int)(W); ++j) strm >> G[i][j];     }     void outputTo(ostream& strm) {       for (int i = 0; i < (int)(H); ++i) {         for (int j = 0; j < (int)(W); ++j) cout << G[i][j];         strm << endl;       }     }   };   template <typename T>   inline istream& operator>>(istream& strm, Grid<T>& g) {     g.scanFrom(strm);     return strm;   }   template <typename T>   inline ostream& operator<<(ostream& strm, Grid<T>& g) {     g.outputTo(strm);     return strm;   }   int n;   int X[1010][1010], Y[1010][1010];   char ans[1010][1010];   bool in(int i, int j) { return 0 <= i && i < n && 0 <= j && j < n; }   int di[4] = {1, 0, -1, 0};   int dj[4] = {0, 1, 0, -1};   int main() {     cin >> n;     for (int i = 0; i < (int)(n); ++i)       for (int j = 0; j < (int)(n); ++j) {         ans[i][j] =  ? ;         scanf( %d %d , &X[i][j], &Y[i][j]);         if (X[i][j] != -1) --X[i][j];         if (Y[i][j] != -1) --Y[i][j];       }     unordered_map<int, unordered_map<int, char>> dir;     dir[1][0] =  D ;     dir[-1][0] =  U ;     dir[0][1] =  R ;     dir[0][-1] =  L ;     function<void(int, int, int, int)> sink = [&](int i, int j, int x, int y) {       for (int k = 0; k < (int)(4); ++k) {         int ni = i + di[k], nj = j + dj[k];         if (!in(ni, nj)) continue;         if (ans[ni][nj] !=  ? ) continue;         if (X[ni][nj] != x || Y[ni][nj] != y) continue;         ans[ni][nj] = dir[i - ni][j - nj];         sink(ni, nj, x, y);       }     };     for (int i = 0; i < (int)(n); ++i)       for (int j = 0; j < (int)(n); ++j) {         if (ans[i][j] !=  ? ) continue;         if (X[i][j] == -1) {           for (int k = 0; k < (int)(4); ++k) {             int ni = i + di[k], nj = j + dj[k];             if (!in(ni, nj)) continue;             if (X[ni][nj] != -1) continue;             ans[i][j] = dir[ni - i][nj - j];             ans[ni][nj] = dir[i - ni][j - nj];             sink(i, j, -1, -1);             sink(ni, nj, -1, -1);           }         } else {           if (ans[X[i][j]][Y[i][j]] !=  ? ) break;           if (X[X[i][j]][Y[i][j]] != X[i][j] || Y[X[i][j]][Y[i][j]] != Y[i][j])             break;           ans[X[i][j]][Y[i][j]] =  X ;           sink(X[i][j], Y[i][j], X[i][j], Y[i][j]);         }       }     for (int i = 0; i < (int)(n); ++i)       for (int j = 0; j < (int)(n); ++j) {         if (ans[i][j] ==  ? ) {           cout <<  INVALID  << endl;           return 0;         }       }     cout <<  VALID  << endl;     for (int i = 0; i < (int)(n); ++i) {       for (int j = 0; j < (int)(n); ++j) cout << ans[i][j];       cout << endl;     }     return 0;   }   