Compute maximum perimeter among islands

## Problem You are given an `m x n` binary grid `grid` where: - `grid[r][c] = 1` represents land - `grid[r][c] = 0` represents water An **island** is a maximal set of land cells connected **4-directionally** (up, down, left, right). The **perimeter** of an island is the total number of unit edges of land cells that either: - lie on the boundary of the grid, or - border a water cell. Return the **maximum perimeter** among all islands in the grid. If there is no land, return `0`. ## Input / Output - **Input:** `grid: List[List[int]]` - **Output:** `int` (maximum island perimeter) ## Constraints (typical interview constraints) - `1 <= m, n <= 1000` (or similar) - `grid[r][c]` is `0` or `1` - Aim for `O(mn)` time ## Example Given: ``` grid = [ [0,1,1,0], [0,1,0,0], [1,1,0,0] ] ``` There are two islands; return the larger island’s perimeter.