DP — Maximal Square

solve(matrix) — a matrix of 0 and 1. Return the area of the largest square
that contains only 1s (0 if there is none).

Let side[r][c] be the side of the largest all-ones square whose **bottom-right
corner** is (r, c). If the cell is 1, then
side = 1 + min(side[r-1][c], side[r][c-1], side[r-1][c-1]); otherwise it is 0.

Examples

solve([[1,0,1,0,0],[1,0,1,1,1],[1,1,1,1,1],[1,0,0,1,0]])  → 4
solve([[0,1],[1,0]])                                      → 1
solve([[0]])                                              → 0
solve([[1,1],[1,1]])                                      → 4