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## Problem

Given a binary tree, return the inorder traversal of its nodes’ values.

Example:

Input: [1,null,2,3]
1
\
2
/
3

Output: [1,3,2]


Follow up: Recursive solution is trivial, could you do it iteratively?

## Code

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {
List<Integer> res;
public List<Integer> inorderTraversal(TreeNode root) {
res = new ArrayList<>();
helper(root);
return res;
}

private void helper(TreeNode root){
if(root == null) return;

helper(root.left);
helper(root.right);
}
}

/**
* 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> res = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
TreeNode curr = root;

while(curr != null || !stack.isEmpty()){
while(curr != null){
stack.push(curr);
curr = curr.left;
}

curr = stack.pop();