https://vjudge.net/problem/UVA-1603
// https://github.com/aoapc-book/aoapc-bac2nd/blob/master/ch7/UVa1603.cpp
// UVa1603 Square Destroyer
// Rujia Liu
// This code implements a variant of an algorithm presented in a book. It's simple yet efficient.
// Readers are encouraged to experiment on other algorithms.
// However, it's still slow for n=5 and m=0 (which is NOT in judge input)
// If you really want an efficient solution, learn DLX (Algorithm X with dancing links)
// DLX is well expained (with code) in my other book <<Beginning Algori