/**
* 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) {
ArrayDeque<TreeNode> stack = new ArrayDeque<>();
List<Integer> list = new LinkedList<>();
if (root == null) return list;
stack.push(root);
while (stack.peek().left != null) {
stack.push(stack.peek().left);
}
while (!stack.isEmpty()) {
TreeNode node = stack.pop();
list.add(node.val);
if (node.right != null) {
stack.push(node.right);
while (stack.peek().left != null) {
stack.push(stack.peek().left);
}
}
}
return list;
}
}