Binary Tree Inorder Traversal_iteratively.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<Integer> result=new ArrayList<Integer>();
        if(root==null) return result; 
        Stack<TreeNode> stack=new Stack<TreeNode>();
        while(!stack.isEmpty()|| root!=null)
        { 
            if(root!=null)
            {
                stack.push(root); root=root.left; 
            }else
            {
                root=stack.pop(); result.add(root.val); root=root.right; 
            }
        }
        return result; 
    }
}

Binary Tree Inorder Traversal_recursively.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(root==null) return new ArrayList<Integer>(); 
        ArrayList<Integer> result=inorderTraversal(root.left); 
        result.add(root.val);
        ArrayList<Integer> right=inorderTraversal(root.right);
        result.addAll(right);
        return result; 
    }
}