func_code_string: #include <bits/stdc++.h>   using namespace std;   int n, x[1500][1500], y[1500][1500], vis[1510][1500];   char ans[1500][1500];   struct node {     int x, y;   };   int dx[] = {0, 0, 1, -1};   int dy[] = {1, -1, 0, 0};   vector<node> a;   int main() {     cin >> n;     for (int i = 1; i <= n; i++) {       for (int j = 1; j <= n; j++) {         scanf( %d%d , &x[i][j], &y[i][j]);       }     }     for (int i = 1; i <= n; i++) {       for (int j = 1; j <= n; j++) {         if (x[i][j] != -1 && !vis[x[i][j]][y[i][j]]) {           if (x[x[i][j]][y[i][j]] == -1 || x[x[i][j]][y[i][j]] != x[i][j] ||               y[x[i][j]][y[i][j]] != y[i][j]) {             printf( INVALID n );             return 0;           }           a.push_back(node{x[i][j], y[i][j]});           ans[x[i][j]][y[i][j]] =  X ;           vis[x[i][j]][y[i][j]] = 1;         }       }     }     queue<node> q;     for (int j = 0; j < a.size(); j++) {       q.push(a[j]);       while (!q.empty()) {         node now = q.front();         q.pop();         for (int i = 0; i < 4; i++) {           int xx = now.x + dx[i];           int yy = now.y + dy[i];           if (xx <= 0 || yy <= 0 || xx > n || yy > n || x[xx][yy] != a[j].x ||               y[xx][yy] != a[j].y)             continue;           if (!vis[xx][yy]) {             vis[xx][yy] = 1;             q.push(node{xx, yy});             if (i == 0) ans[xx][yy] =  L ;             if (i == 1) ans[xx][yy] =  R ;             if (i == 2) ans[xx][yy] =  U ;             if (i == 3) ans[xx][yy] =  D ;           }         }       }     }     for (int i = 1; i <= n; i++) {       for (int j = 1; j <= n; j++) {         if (x[i][j] != -1 && !vis[i][j]) {           printf( INVALID n );           return 0;         }         if (x[i][j] == -1 && !vis[i][j]) {           q.push(node{i, j});           vis[i][j] = 1;           int flag = 0;           while (!q.empty()) {             node now = q.front();             q.pop();             for (int k = 0; k < 4; k++) {               int xx = now.x + dx[k];               int yy = now.y + dy[k];               if (xx <= 0 || yy <= 0 || xx > n || yy > n || x[xx][yy] != -1)                 continue;               if (x[xx][yy] == -1 && vis[xx][yy]) {                 if (!flag) {                   if (k == 0) ans[i][j] =  R ;                   if (k == 1) ans[i][j] =  L ;                   if (k == 2) ans[i][j] =  D ;                   if (k == 3) ans[i][j] =  U ;                   flag = 1;                 }               }               if (!vis[xx][yy]) {                 vis[xx][yy] = 1;                 q.push(node{xx, yy});                 if (k == 0) ans[xx][yy] =  L ;                 if (k == 1) ans[xx][yy] =  R ;                 if (k == 2) ans[xx][yy] =  U ;                 if (k == 3) ans[xx][yy] =  D ;                 if (!flag) {                   if (k == 0) ans[i][j] =  R ;                   if (k == 1) ans[i][j] =  L ;                   if (k == 2) ans[i][j] =  D ;                   if (k == 3) ans[i][j] =  U ;                   flag = 1;                 }               }             }             if (!flag) {               printf( INVALID n );               return 0;             }           }         }       }     }     printf( VALID n );     for (int i = 1; i <= n; i++) {       for (int j = 1; j <= n; j++) {         printf( %c , ans[i][j]);       }       cout << endl;     }   }   