# 1139. Largest 1-Bordered Square

https://leetcode.com/problems/largest-1-bordered-square

## Description

Given a 2D `grid` of `0`s and `1`s, return the number of elements in the largest **square** subgrid that has all `1`s on its **border**, or `0` if such a subgrid doesn't exist in the `grid`.

**Example 1:**

```

**Input:** grid = [[1,1,1],[1,0,1],[1,1,1]]
**Output:** 9
```

**Example 2:**

```

**Input:** grid = [[1,1,0,0]]
**Output:** 1
```

**Constraints:**

* `1 <= grid.length <= 100`
* `1 <= grid[0].length <= 100`
* `grid[i][j]` is `0` or `1`

## ac

```java
```
