Leetcode 94 Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

Solution 1: recursive

 public class Solution {  
   public List<Integer> inorderTraversal(TreeNode root) {  
     List<Integer> res=new ArrayList<>();  
     travel(root, res);  
     return res;  
   }  
   private void travel(TreeNode x, List<Integer> res){  
     if (x==null) return;  
     travel(x.left,res);  
     res.add(x.val);  
     travel(x.right,res);  
   }  
 }  

Solution 2: iterative

 public class Solution {  
   public List<Integer> inorderTraversal(TreeNode root) {  
     Deque<TreeNode> stack=new LinkedList<>();  
     List<Integer> res=new ArrayList<>();  
     TreeNode x=root;  
     while (x!=null || !stack.isEmpty()) {  
       if (x!=null) {  
         stack.push(x);  
         x=x.left;  
       }  
       else {  
         x=stack.pop();  
         res.add(x.val);  
         x=x.right;  
       }  
     }  
     return res;  
   }  
 }