非遞歸（疊代）實作二叉樹的中序周遊

/**
 * 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 {
        Stack<TreeNode> stack = new Stack<>();
        ArrayList<Integer> list = new ArrayList<>();
        public List<Integer> inorderTraversal(TreeNode root) {
            if (root == null) return list;
            while (!stack.isEmpty() || root != null){
                while(root != null){
                    stack.push(root);       //先将樹頂端的壓入棧中
                    root = root.left;       //循環結束後，棧頂元素是最左端的元素，也符合中序周遊的思想
                }
                //其實在循環過程中，已經将路徑上的中間結點壓入了棧中
                TreeNode node = stack.pop();//彈出最左端元素
                list.add(node.val);            //加傳入連結表中
                if (node.right != null){    //如果目前結點右右子樹，那麼右子樹也要進行一樣的處理
                    root = node.right;
                }
            }
            return list;
        }
    }