0463-island-perimeter.cpp

// Time complexity is O(M*N)
// Space complexity is O(1)

class Solution {
public:
    int islandPerimeter(vector<vector<int>>& grid) {
      int m = grid.size();
      int n = grid[0].size();
      
      int prem = 0;
      
      for(int i = 0 ; i < m ; i++)
      {
        for(int j = 0 ; j < n ; j++)
        {
          if(grid[i][j] == 1) {
            prem += 4;
          
            if(j > 0 && grid[i][j-1] == 1) prem -=2;
            if(i >0 && grid[i-1][j] == 1) prem -=2;

          }
        }
      }
      
      return prem;
    }
};