Preorder, middle order and postorder traversal of binary tree

Input {1,2,3}
Output
[[1,2,3],[2,1,3],[2,3,1]]

java

public class Solution {
    /**
     * 
     * @param root TreeNode class  the root of binary tree
     * @return int Two dimensional arrays of integers 
     */
    List<Integer> front = new ArrayList<>();
    List<Integer> mid = new ArrayList<>();
    List<Integer> back = new ArrayList<>();
    public int[][] threeOrders (TreeNode root) {
        // write code here
        PreorderTraversal(root);
        InorderTraversal(root);
        PostorderTraversal(root);
         
        int[][] res = {
            front.stream().mapToInt(Integer::valueOf).toArray(),
            mid.stream().mapToInt(Integer::valueOf).toArray(),
            back.stream().mapToInt(Integer::valueOf).toArray()
        };
         
        return res;
    }
     
    // The former sequence traversal 
    public void PreorderTraversal(TreeNode root){
        if(root == null)
            return;
        front.add(root.val);
        PreorderTraversal(root.left);
        PreorderTraversal(root.right);
    }
     
    // In the sequence traversal 
    public void InorderTraversal(TreeNode root){
        if(root == null)
            return;
        InorderTraversal(root.left);
        mid.add(root.val);
        InorderTraversal(root.right);
    }
     
    // After the sequence traversal 
    public void PostorderTraversal(TreeNode root){
        if(root == null)
            return;
        PostorderTraversal(root.left);
        PostorderTraversal(root.right);
        back.add(root.val);
    }
}