求解释代码，添加注释或者画图解释（在网上看到的求组合的递归算法，绕晕头了看不懂，求解释）

public class Test1 {

public static void main(String[] args) {
select(2);
}

private static void select(int k) {
char[] result = new char[k];
subselect(0, 1, result, k);

}

private static void subselect(int head, int index, char[] r, int k) {
for (int i = head; i < a.length + index - k; i++) {
if (index < k) {
r[index - 1] = a[i];
System.out.println("i="+(i)+";index="+(index));
subselect(i + 1, index + 1, r, k);
} else if (index == k) {
r[index - 1] = a[i];
System.out.println(";i="+(i)+";index="+(index)+";index==k:"+(index==k));
System.out.print(i+"===");
System.out.println(r);
subselect(i + 1, index + 1, r, k);
} else {
System.out.println("++");
return;//返回到何处？奇怪
}

}
}

private static char[] a = { 'a', 'b', 'c' };
}