Binary Tree Inorder Traversal

 1 public class Solution {
 2     List<Integer> result = new ArrayList<Integer>();
 3  
 4     public List<Integer> inorderTraversal(TreeNode root) {
 5         if(root !=null){
 6             helper(root);
 7         }
 8  
 9         return result;
10     }
11  
12     public void helper(TreeNode p){
13         if(p.left!=null)
14             helper(p.left);
15  
16         result.add(p.val);
17  
18         if(p.right!=null)
19             helper(p.right);
20     }
21 }

 1 public class Solution {
 2     public ArrayList<Integer> inorderTraversal(TreeNode root) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5          ArrayList<Integer> lst = new ArrayList<Integer>();
 6  
 7         if(root == null)
 8             return lst; 
 9  
10         Stack<TreeNode> stack = new Stack<TreeNode>();
11         //define a pointer to track nodes
12         TreeNode p = root;
13  
14         while(!stack.empty() || p != null){
15  
16             // if it is not null, push to stack
17             //and go down the tree to left
18             if(p != null){
19                 stack.push(p);
20                 p = p.left;
21  
22             // if no left child
23             // pop stack, process the node
24             // then let p point to the right
25             }else{
26                 TreeNode t = stack.pop();
27                 lst.add(t.val);
28                 p = t.right;
29             }
30         }
31  
32         return lst;
33     }
34 }