public class Solution {    private boolean[][] visited = new boolean[3][3];    private int m;    private int n;    public int numberOfPatterns(int m, int n) {        if (n == 0 || m > n) {            return 0;        }        int result = 0;        this.m = m;        this.n = n;        for (int i = 0; i < 3; i++) {            for (int j = 0; j < 3; j++) {                visited[i][j] = true;                result += getPattern(i, j, 1);                visited[i][j] = false;            }        }        return result;    }            private int getPattern(int x, int y, int level) {        if (level > n) {            return 0;        }                int result = level >= m ? 1 : 0;        for (int i = 0; i < 3; i++) {            for (int j = 0; j < 3; j++) {                if (visited[i][j] ||                x == i && Math.abs(y - j) == 2 && !visited[i][1] ||                y == j && Math.abs(x - i) == 2 && !visited[1][j] ||                Math.abs(x - i) == 2 && Math.abs(y - j) == 2 && !visited[1][1]) {                    continue;                }                visited[i][j] = true;                result += getPattern(i, j, level + 1);                visited[i][j] = false;            }        }        return result;    }}

 

 

1. 3x3 pattern board. Do not mess it up.

2. pattern counting start from 1, not 0