InOrder.java

import java.util.*;

public class InOrder {
    public void traversal(TreeNode root, List<Integer> res) {
        if (root == null) {
            return;
        }
        traversal(root.left, res);
        res.add(root.val);
        traversal(root.right, res);
    }

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

    public List<Integer> inorder(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        while (true) {
            if (curr != null) {
                stack.push(curr);
                curr = curr.left;
            } else {
                if (stack.isEmpty()) {
                    break;
                }
                curr = stack.pop();
                res.add(curr.val);
                curr = curr.right;
            }
        }
        return res;
    }
}