Question

What does this function do? And how do you evaluate it?

How to trace it? How to understand recursive method?

I just can’t understand the recursion, and the exam date is coming soon, and I know in order to understand recursion, I must first understand recursion.

But I just can’t write a slightly complex recursive method, can anyone help me out using the simplest English words.

public class BalancedStrings {

    public static void printBalanced(String prefix, int a, int b) {
        if (a > 0)
            printBalanced(prefix + "a", a - 1, b);
        if (b > 0)
            printBalanced(prefix + "b", a, b - 1);
        if (a == 0 && b == 0)
            System.out.println(prefix);
    }

    public static void printBalanced(int n) {
        if (n % 2 == 0) {
            printBalanced("", n / 2, n / 2);
        }
    }

    public static void main(String[] args) {
        printBalanced(4);
    }
}

Answer 1

The calls to printBalanced() are the recursive calls. The way to recognize recursion is when a function is calling itself. Recursion is best understood when drawn using a tree:

The branches of the tree will continue to create more functions until an end condition is met, which in this case is a == 0 && b == 0. The function you provided looks like it is printing the string ‘prefix’ concatenated with a specified number of ‘a’ and ‘b’ characters, recursively. When the variables a and b reach 0, the recursion stops and the result is printed using System.out.println(prefix);

In the main function you are passing the integer 4 to printBalanced(int n) which the calls printBalanced(String prefix, int a, int b) with parameters like this: printBalanced("",2,2);

The overall result is prefix concatenated with a balanced number of a’s and b’s