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  1. #include <bits/stdc++.h>
  2. using namespace std;
  3.  
  4. using ll = long long;
  5.  
  6. int n, m;
  7. vector<vector<int>> grid;
  8. vector<vector<int>> vis;
  9.  
  10. int dx[] = {0, 0, 1, -1};
  11. int dy[] = {1, -1, 0, 0};
  12.  
  13. queue<pair<int,int>> q;
  14.  
  15. // DFS to mark the first island
  16. void dfs(int x, int y){
  17.     vis[x][y] = 1;
  18.     grid[x][y] = 2;          // mark first island
  19.  
  20.     q.push({x, y});          // every cell becomes a BFS source
  21.  
  22.     for(int k = 0; k < 4; k++){
  23.         int nx = x + dx[k];
  24.         int ny = y + dy[k];
  25.  
  26.         if(nx >= 0 && nx < n && ny >= 0 && ny < m &&
  27.            !vis[nx][ny] && grid[nx][ny] == 1){
  28.             dfs(nx, ny);
  29.         }
  30.     }
  31. }
  32.  
  33. int main(){
  34.  
  35.     ios::sync_with_stdio(false);
  36.     cin.tie(nullptr);
  37.  
  38.     cin >> n >> m;
  39.  
  40.     grid.assign(n, vector<int>(m));
  41.     vis.assign(n, vector<int>(m, 0));
  42.  
  43.     for(int i = 0; i < n; i++)
  44.         for(int j = 0; j < m; j++)
  45.             cin >> grid[i][j];
  46.  
  47.     // Find first island
  48.     bool found = false;
  49.  
  50.     for(int i = 0; i < n && !found; i++){
  51.         for(int j = 0; j < m && !found; j++){
  52.             if(grid[i][j] == 1){
  53.                 dfs(i, j);
  54.                 found = true;
  55.             }
  56.         }
  57.     }
  58.  
  59.     vector<vector<int>> dist(n, vector<int>(m, 0));
  60.  
  61.     // Multi-source BFS
  62.     while(!q.empty()){
  63.  
  64.         auto [x, y] = q.front();
  65.         q.pop();
  66.  
  67.         for(int k = 0; k < 4; k++){
  68.  
  69.             int nx = x + dx[k];
  70.             int ny = y + dy[k];
  71.  
  72.             if(nx < 0 || nx >= n || ny < 0 || ny >= m)
  73.                 continue;
  74.  
  75.             // Reached second island
  76.             if(grid[nx][ny] == 1){
  77.                 cout << dist[x][y] << '\n';
  78.                 return 0;
  79.             }
  80.  
  81.             // Expand only through water
  82.             if(grid[nx][ny] == 0){
  83.  
  84.                 grid[nx][ny] = 2;      // mark visited
  85.                 dist[nx][ny] = dist[x][y] + 1;
  86.                 q.push({nx, ny});
  87.             }
  88.         }
  89.     }
  90.  
  91.     return 0;
  92. }
Success #stdin #stdout 0s 5328KB
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stdin
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4 4
1 0 0 0
0 0 0 0
0 0 0 0
0 0 0 1
stdout
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5
https://ideone.com/LqNMrS
language:
C++ (gcc 8.3)
created:
18 hours ago
visibility:
public

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