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  1. #include <iostream>
  2. #include <vector>
  3. #include <numeric>
  4. using namespace std;
  5. struct DSU {
  6.     vector<int> parent;
  7.  
  8.     DSU(int n) {
  9.         parent.resize(n);
  10.         iota(parent.begin(), parent.end(), 0);
  11.     }
  12.  
  13.     int find_set(int v) {
  14.         if (v == parent[v])
  15.             return v;
  16.         return parent[v] = find_set(parent[v]); 
  17.     }
  18.     void union_sets(int a, int b) {
  19.         a = find_set(a);
  20.         b = find_set(b);
  21.         if (a != b) {
  22.             parent[b] = a;
  23.         }
  24.     }
  25. };
  26.  
  27. int main() {
  28.     ios_base::sync_with_stdio(false);
  29.     cin.tie(NULL);
  30.  
  31.     int n, m;
  32.     if (!(cin >> n >> m)) return 0;
  33.  
  34.     vector<vector<int>> matrix(n, vector<int>(n, 0));
  35.     int virtual_top = n * n;
  36.     int virtual_bottom = n * n + 1;
  37.  
  38.     DSU dsu(n * n + 2);
  39.  
  40.     int dr[] = {-1, 1, 0, 0};
  41.     int dc[] = {0, 0, -1, 1};
  42.  
  43.     int result = -1;
  44.  
  45.     for (int i = 1; i <= m; i++) {
  46.         int r, c;
  47.         cin >> r >> c;
  48.         if (result != -1) continue;
  49.  
  50.         matrix[r][c] = 1;
  51.         int current_id = r * n + c; 
  52.  
  53.         if (r == 0) {
  54.             dsu.union_sets(current_id, virtual_top);
  55.         }
  56.         if (r == n - 1) {
  57.             dsu.union_sets(current_id, virtual_bottom);
  58.         }
  59.         for (int d = 0; d < 4; d++) {
  60.             int nr = r + dr[d];
  61.             int nc = c + dc[d];
  62.             if (nr >= 0 && nr < n && nc >= 0 && nc < n && matrix[nr][nc] == 1) {
  63.                 int neighbor_id = nr * n + nc;
  64.                 dsu.union_sets(current_id, neighbor_id);
  65.             }
  66.         }
  67.         if (dsu.find_set(virtual_top) == dsu.find_set(virtual_bottom)) {
  68.             result = i;
  69.         }
  70.     }
  71.     cout << result << "\n";
  72.     return 0;
  73. }
Success #stdin #stdout 0.01s 5316KB
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https://ideone.com/LN2fE5
language:
C++ (gcc 8.3)
created:
1 day ago
visibility:
public

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