#517 · #351 · Android Unlock Patterns(安卓系统手势解锁)

Android unlock patterns are drawn on a 3x3 grid of dots using 4 to 9 distinct dots. A move may skip over an already-visited dot, but cannot jump over an unvisited one. Given m and n, return the number of valid patterns of length between m and n inclusive. Constraints: 1 <= m, n <= 9.