94. Binary Tree Inorder Traversal

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?

public List<Integer> inorderTraversal(TreeNode root) {
    Stack<TreeNode> stack = new Stack<>();
    TreeNode curr = root;
    ArrayList<Integer> result = new ArrayList<>();
    while(curr != null || !stack.isEmpty()) {
        while(curr != null) {
            stack.push(curr);
            curr = curr.left;
        }
        curr = stack.pop();
        result.add(curr.val);
        curr = curr.right;
    }
    return result;
}