Iterative version
- Use Stack
- Return empty list if the tree is null
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> ans = new ArrayList<>();
if (root == null) {
return ans;
}
Deque<TreeNode> deque = new Deque<>();
deque.addFirst(root);
while(!deque.isEmpty()) {
TreeNode
}
}
}