二叉树中序遍历，分为递归和非递归方法：

递归：

private static List
     
       inList = new ArrayList<>();public static List
      
        inorderTraversalRec(TreeNode root){    if (null == root){        return inList;    }    inorder(root);    return inList;}private static void inorder(TreeNode root){    if (null == root){        return;    }    inorder(root.left);    inList.add(root.val);    inorder(root.right);}
      
     

非递归：

public static List
     
       inorderTraversal(TreeNode root) {    List
      
        list = new ArrayList<>();    if (null == root){        return list;    }    Stack
       
         stack = new Stack<>();    TreeNode cur = root;    while (null != cur || !stack.isEmpty()){        while (null != cur){            stack.push(cur);            cur = cur.left;        }        TreeNode node = stack.pop();        list.add(node.val);        cur = node.right;    }    return list;}