Alphabetic string

 title ： A string of letters 

 from  A,B,C  this 3 One letter can make up a lot of strings .
 such as ："A","AB","ABC","ABA","AACBB" ....

 Now? , Xiao Ming is thinking about a problem ：
 If there is a limit to the number of letters per letter , How many strings of known length can be formed ？

 He asked his good friend to help , And soon I got the code ,
 The solution is super simple , The most important part, however, is vague .

 Please analyze the source code carefully , Fill in what is missing in the underlined section .


public class A
{
	// a individual A,b individual B,c individual C  Letter , How many different lengths can be made of n String of .
	static int f(int a, int b, int c, int n)
	{
		if(a<0 || b<0 || c<0) return 0;
		if(n==0) return 1; 
		
		return ________________________________;  // Fill in the blanks 
	}
	
	public static void main(String[] args)
	{
		System.out.println(f(1,1,1,2));
		System.out.println(f(1,2,3,3));
	}
}

 For the above test data , The result of Xiaoming's mental arithmetic should be ：
6
19


 Be careful ： Just fill in the missing code in the underlined part , Don't submit anything superfluous or descriptive .

answer ： At the first sight of the topic , My first idea is multiple permutations in combinatorics ,

seek r1 individual 1,r2 individual 2,…,rt individual t Number of permutations of , set up r1+r2+…+rt=n,

The number of multiple full permutations is P(n;r1,r2,…,rt).

But in fact, it's complicated to think so , No brain recursion .

 title ： A string of letters 

 from  A,B,C  this 3 One letter can make up a lot of strings .
 such as ："A","AB","ABC","ABA","AACBB" ....

 Now? , Xiao Ming is thinking about a problem ：
 If there is a limit to the number of letters per letter , How many strings of known length can be formed ？

 He asked his good friend to help , And soon I got the code ,
 The solution is super simple , The most important part, however, is vague .

 Please analyze the source code carefully , Fill in what is missing in the underlined section .


public class A
{
	// a individual A,b individual B,c individual C  Letter , How many different lengths can be made of n String of .
	static int f(int a, int b, int c, int n)
	{
		if(a<0 || b<0 || c<0) return 0;
		if(n==0) return 1; 
		
		return f(a-1,b,c,n-1)+f(a,b-1,c,n-1)+f(a,b,c-1,n-1);  // Fill in the blanks 
	}
	
	public static void main(String[] args)
	{
		System.out.println(f(1,1,1,2));
		System.out.println(f(1,2,3,3));
	}
}

 For the above test data , The result of Xiaoming's mental arithmetic should be ：
6
19


 Be careful ： Just fill in the missing code in the underlined part , Don't submit anything superfluous or descriptive .