Inorder Traversal Recursive

InorderTraversalRecursive.java

package com.interviewbytes.trees;

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

public class InorderTraversalRecursive {

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

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

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

TreeNode.java

package com.interviewbytes.trees;

public class TreeNode {

    int val;
    TreeNode left;
    TreeNode right;

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