Recursive and non-recursive

 1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     12     public List<Integer> inorderTraversal(TreeNode root) {13         ArrayList<Integer> arr=new ArrayList<Integer>();14         if(root==null) return arr;15        inorder(root,arr);16        return (List)arr;17         18         19     }20      public void inorder(TreeNode root,ArrayList<Integer> arr) {21         if(root!=null)22         {23            inorder(root.left,arr);24            arr.add(root.val);25            inorder(root.right,arr);26         }27 28         29         30     }31 }

2. Non-recursion

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {        public List<Integer> inorderTraversal(TreeNode root) {        ArrayList<Integer> arr=new ArrayList<Integer>();        if(root==null) return arr;       Stack<TreeNode> s=new Stack<TreeNode>();       TreeNode t=root;       while(!s.isEmpty()||t!=null)       {           while(t!=null)           {               s.push(t);               t=t.left;           }                      if(s!=null)           {               t=s.pop();              arr.add(t.val);              t=t.right;                                         }                             }                return arr;    }    }