94. Binary Tree Inorder Traversal

Iterative version

1. Use Stack
2. 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 inorderTraversal(TreeNode root) {
        List ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        Deque deque = new Deque<>();
        deque.addFirst(root);
        while(!deque.isEmpty()) {
            TreeNode
        }
    }
    
}