Largest Connected Region

You are given an m x n binary matrix grid, where every cell is $0$ or $1$.

A region is a group of $1$ cells connected 4-directionally: up, down, left, or right. Cells touching only at a corner are not connected. The size of a region is how many cells it contains.

Return the size of the largest region in grid, or $0$ if the grid contains no $1$ at all.

Examples

Example 1

Input: grid = [[0,0,1,0,0,0,0,1,0,0,0,0,0],
               [0,0,0,0,0,0,0,1,1,1,0,0,0],
               [0,1,1,0,1,0,0,0,0,0,0,0,0],
               [0,1,0,0,1,1,0,0,1,0,1,0,0],
               [0,1,0,0,1,1,0,0,1,1,1,0,0],
               [0,0,0,0,0,0,0,0,0,0,1,0,0],
               [0,0,0,0,0,0,0,1,1,1,0,0,0],
               [0,0,0,0,0,0,0,1,1,0,0,0,0]]
Output: 6
Explanation: The largest region is the block of six 1s in rows 3 and 4, columns 4 and 5
plus rows 2 and 3, column 4. No region has more than six cells. Note the 1 at row 0,
column 2 touches nothing, so it forms a region of size 1 on its own.

Example 2

Input: grid = [[0,0,0,0,0,0,0,0]]
Output: 0
Explanation: There is no 1 anywhere, so there is no region.