BinaryTreeInorderTraversal.java

/**
 * 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> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode node = root;
        
        while (node != null || !stack.isEmpty())
        {
            if (node != null)
            {
                stack.push(node);
                node = node.left;
            }
            else
            {
                node = stack.pop();
                list.add(node.val);
                node = node.right;
            }
        }
        
        return list;
    }
}