Tree amplitude - study code

TreeAmplitude_studyCode.java

import java.util.*;
class TreeNode
{
    public TreeNode left, right;
    public int  val;
    public TreeNode(String val)
    {
        this.val = Integer.parseInt(val);
    }
}
public class Solution {
 
    public static TreeNode buildTree(String[] input)
    {
        int index = 0;
        TreeNode root = new TreeNode(input[index++]);
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(index < input.length)
        {
             
            int size = queue.size();
            for(int i = 0; i< size; i++)
            {
                TreeNode t = queue.poll();
                if(index >= input.length) return root;
                if(!input[index].equals("*"))
                {
                    t.left = new TreeNode(input[index]);
                    queue.offer(t.left);
                }
                index++;
                if(index >= input.length) return root;
                if(!input[index].equals("*"))
                {
                    t.right = new TreeNode(input[index]);
                    queue.offer(t.right);
                }
                index++;
                if(index >= input.length) return root;
            }
        }
        return root;
    }
     
    public static int amplitude(TreeNode root)
    {
        if(root == null) return 0;
        return DFS(root, Integer.MAX_VALUE, Integer.MIN_VALUE);
    }
    private static int DFS(TreeNode root, int min, int max)
    {
        if(root == null) return max - min;
        if(root.val < min) min = root.val;
        if(root.val > max) max = root.val;
        return Math.max(DFS(root.left, min, max), DFS(root.right, min, max));
         
    }
 
    public static void main(String[] args)
    {
        String[] input = {"5", "-8", "9", "-12", "2", "8", "4", "*", "*", "*", "*", "2", "*", "5"};
        TreeNode root = buildTree(input);
        System.out.println(amplitude(root));
    }
}