Enumeration of all combinations Java

When I was solving the problem that I wanted to get all the combinations of characters from the given character string, I looked it up and found that chakotay's blog = 612) had what I was looking for !!!

	public static void permutation(String q, String ans){
		// Base Case
		if(q.length() <= 1) {
			System.out.println(ans + q);
		}
		// General Case
		else {
			for (int i = 0; i < q.length(); i++) {
				permutation(q.substring(0, i) + q.substring(i + 1),
						ans + q.charAt(i));
			}
		}
	}

I was impressed by the very concise code, even though I searched various things including English !!! But I don't know what's going on. .. .. Therefore, I would like to look at recursion step by step. As an example, let's take a look at the behavior of permutation ("abc", "") in order.

permutation("abc","")
//q.length=3 so go to else
//i=When 0(This i is less than 3)
	permutation("bc","a")
	//q.length=2 so go to else
	//i=When 0(This i is less than 2)
		permutation("c","ab")
		//q.length=1 so go to if
		System.out.println("abc");
	//i=When 1(This i is less than 2)
		permutation("b","ac")
		//q.length=1 so go to if
		System.out.println("acb");
	//permutation("bc","a") done	
//i=When 1(This i is less than 3)
	permutation("ac","b")
	//q.length=2 so go to else
	//i=When 0(This i is less than 2)
		permutation("c","ba")
		//q.length=1 so go to if
		System.out.println("bac");
	//i=When 1(This i is less than 2)
		permutation("a","bc")
		//q.length=1 so go to if
		System.out.println("bca");
	//permutation("ac","b") done	
//i=When 2(This i is less than 3)
	permutation("ab","c")
	//q.length=2 so go to else
	//i=When 0(This i is less than 2)
		permutation("b","ca")
		//q.length=1 so go to if
		System.out.println("cab");
	//i=When 1(This i is less than 2)
		permutation("a","cb")
		//q.length=1 so go to if
		System.out.println("cba");
	//permutation("ab","c") done	
//permutation("abc","") done

It is like this. Is it difficult to understand? I wrote it down and understood it, but I often admired that I couldn't come up with such a method on my own.

that's all. It was my first post.