递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
   public List inorderTraversal(TreeNode root) {
        List ls = new LinkedList();
         mid(root,ls);
        return ls;
    }
    public static void mid(TreeNode T,List ls){
        if(T!=null){
            mid(T.left,ls);
            ls.add(T.val);
            mid(T.right,ls);
        }
    }
}

递推

public List inorderTraversal(TreeNode root) {
        List ls = new LinkedList();
        Stack s = new Stack();
        TreeNode temp = root;
        while(temp!=null||!s.isEmpty()) {
            while(temp!=null) {
                s.push(temp);
                temp = temp.left;
            }
            temp = s.pop();                                                      //感觉这个是左节点 其实相对的话是中   容易产生错觉
            ls.add(temp.val);
            temp = temp.right;
        }
        return ls;
    }
}