You are given a 2D grid of '0' and '1' characters.
Find the biggest rectangle made up of only 1s and return its area.
Input: grid = [ ["1","1","0","1"], ["1","1","1","1"], ["1","1","1","0"] ]
Output: 6
Explanation:
The largest rectangle of only 1s is 2 rows × 3 columns = area 6.
Input: grid = [["0","0"]]
Output: 0
Explanation:
No rectangle of 1s exists.
Input: grid = [["1","1"],["1","1"]]
Output: 4
Explanation:
All cells are 1 → rectangle size is 2 × 2 = 4.
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