LeetCode. Binary Tree Inorder Traversal recursive, iterative

BinaryTreeInorderTraversal.java

import java.util.*;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BinaryTreeInorderTraversal {

    // recursive
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<>();
        if(root == null) return ret;

        if(root.left != null)
            ret.addAll(inorderTraversal(root.left));
        ret.add(root.val);
        if(root.right != null)
            ret.addAll(inorderTraversal(root.right));

        return ret;
    }

    // iterative
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<>();
        if(root == null) return ret;

        Stack<TreeNode> st = new Stack<>();
        TreeNode p = root;
        while(!st.isEmpty() || p != null){
            if(p != null){
                st.push(p);
                p = p.left;
            }else{
                TreeNode n = st.pop();
                ret.add(n.val);
                p = n.right;
            }
        }
        return ret;
    }
}