二叉树中序遍历非递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ls=new ArrayList<Integer>();
        if(root == null)
        {return ls;}
        Stack<TreeNode> st=new  Stack<TreeNode>();
        HashSet<TreeNode> hs = new HashSet<TreeNode>();
        st.push(root);
        while(!st.isEmpty())
        {
            TreeNode temp=st.pop();
            if(hs.contains(temp))
            {
                ls.add(temp.val);
                continue;
            }
            hs.add(temp);
            
            if(temp.right!=null)
            {
                st.push(temp.right);
            }
            st.push(temp);
                      if(temp.left!=null)
            {
                st.push(temp.left);
            }
        }
        return ls;
    }
}