Preorder Recursive

PreorderRecursive.java

package com.interviewbytes.trees;


import java.util.ArrayList;
import java.util.List;

public class PreorderRecursive {

    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        helper(root, list);
        return list;
    }

    private void helper(TreeNode node, List<Integer> list) {
        if (node == null) return;

        list.add(node.val);
        helper(node.left, list);
        helper(node.right, list);
    }
}

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}