Java で再帰関数を使用して問題を解決する方法

public class Fibonacci {
    public static int fibonacci(int n) {
        if (n <= 0) {
            return 0;
        }
        if (n == 1 || n == 2) {
            return 1;
        }
        return fibonacci(n - 1) + fibonacci(n - 2);
    }

    public static void main(String[] args) {
        int n = 10;
        System.out.println("斐波那契数列的第" + n + "项是：" + fibonacci(n));
    }
}

public class Factorial {
    public static int factorial(int n) {
        if (n <= 0) {
            return 1;
        }
        return n * factorial(n - 1);
    }

    public static void main(String[] args) {
        int n = 5;
        System.out.println(n + "的阶乘是：" + factorial(n));
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int val) {
        this.val = val;
    }
}

public class BinaryTree {
    public static void preorderTraversal(TreeNode root) {
        if (root != null) {
            System.out.print(root.val + " ");
            preorderTraversal(root.left);
            preorderTraversal(root.right);
        }
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);

        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);

        System.out.println("二叉树的前序遍历结果是：");
        preorderTraversal(root);
    }
}