/**

* Definition for a binary tree node.

* public class TreeNode {

* int val;

* TreeNode left;

* TreeNode right;

* TreeNode() {}

* TreeNode(int val) { this.val = val; }

* TreeNode(int val, TreeNode left, TreeNode right) {

* this.val = val;

* this.left = left;

* this.right = right;

* }

* }

*/

class Solution {

public List<Integer> inorderTraversal(TreeNode root) {

inorder(root);

return result;

}

private final List<Integer> result = new ArrayList<>();

private void inorder(TreeNode p) {

if(p == null) return;

if(p.left != null) inorder(p.left);

result.add(p.val);

if(p.right != null) inorder(p.right);

}

}

class Solution {

public List<Integer> inorderTraversal(TreeNode root) {

if(root == null) return Collections.emptyList();

Deque<TreeNode> st = new ArrayDeque<>();

TreeNode p = root;

List<Integer> result = new ArrayList<>();

while(p != null || !st.isEmpty()){

if(p != null) {

st.push(p);

p = p.left;

}else{

TreeNode t = st.pop();

result.add(t.val);

if(t.right != null) p = t.right;

}

}

return result;

}

}