94. Binary Tree Inorder Traversal

 1 //Recursive
 2 class Solution {
 3        List<Integer> res = new ArrayList<>();
 4     public List<Integer> inorderTraversal(TreeNode root) {
 5         if (root != null) {
 6             inorderTraversal(root.left);
 7             res.add(root.val);
 8             inorderTraversal(root.right);
 9         }
10         return res;
11         
12         
13     }
14 }
15 
16 //Iterate
17 class Solution {
18        List<Integer> res = new ArrayList<>();
19     public List<Integer> inorderTraversal(TreeNode root) {
20         if(root == null) return res;
21         Stack<TreeNode> stack = new Stack<>();
22         stack.push(root);
23         Set<TreeNode> visited = new HashSet<>(); 
24         while(!stack.empty()) {
25             TreeNode node = stack.pop();
26             while(node.left != null && !visited.contains(node)) {
27                 visited.add(node);
28                 stack.push(node);
29                 node = node.left;
30             }
31             res.add(node.val);
32             if(node.right != null) {
33                 stack.push(node.right);
34             }
35             
36         }
37         return res;  
38     }
39 }