迭代解答：模拟递归的入栈出栈操作

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    ArrayList<Integer> res = new ArrayList<>();
    public int[] inorderTraversal (TreeNode root) {
        // write code here
        //递归解法
        if(root==null) return new int[0];
//         dfs(root);
        //迭代解法
        Stack<TreeNode> sk = new Stack<>();
        //sk.push(root);
        //先将左子树全部加入栈中
        while(root!=null || !sk.isEmpty()){
            while(root!=null){
                sk.push(root);
                root = root.left;
            }
            root = sk.pop();
            res.add(root.val);
            root = root.right;
        }
        int len = res.size();
        int[] result = new int[len];
        for(int i = 0;i < len;i++){
            result[i] = res.get(i);
        }
        return result;
    }
}