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463. Island Perimeter 👍

  • Time: $O(mn)$
  • Space: $O(1)$
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class Solution {
 public:
  int islandPerimeter(vector<vector<int>>& grid) {
    int islands = 0;
    int neighbors = 0;

    for (int i = 0; i < grid.size(); ++i)
      for (int j = 0; j < grid[0].size(); ++j)
        if (grid[i][j]) {
          ++islands;
          if (i - 1 >= 0 && grid[i - 1][j])
            ++neighbors;
          if (j - 1 >= 0 && grid[i][j - 1])
            ++neighbors;
        }

    return islands * 4 - neighbors * 2;
  }
};

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class Solution {
  public int islandPerimeter(int[][] grid) {
    int islands = 0;
    int neighbors = 0;

    for (int i = 0; i < grid.length; ++i)
      for (int j = 0; j < grid[0].length; ++j)
        if (grid[i][j] == 1) {
          ++islands;
          if (i - 1 >= 0 && grid[i - 1][j] == 1)
            ++neighbors;
          if (j - 1 >= 0 && grid[i][j - 1] == 1)
            ++neighbors;
        }

    return islands * 4 - neighbors * 2;
  }
}

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class Solution:
  def islandPerimeter(self, grid: list[list[int]]) -> int:
    m = len(grid)
    n = len(grid[0])

    islands = 0
    neighbors = 0

    for i in range(m):
      for j in range(n):
        if grid[i][j] == 1:
          islands += 1
          if i + 1 < m and grid[i + 1][j] == 1:
            neighbors += 1
          if j + 1 < n and grid[i][j + 1] == 1:
            neighbors += 1

    return islands * 4 - neighbors * 2