Largest Plus Sign

Given an n×n grid of 1s with some cells set to 0 (mines), find the largest order k of an axis-aligned plus sign made of 1s (a center with k−1 consecutive 1s up/down/left/right). The core challenge is to compute, for each cell, the consecutive 1-run lengths in all four directions (via four directional DP/scans) and take the maximum of the cell-wise minima, achievable in O(n^2) time.