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# 01/03/2022 144

144\. Binary Tree Preorder Traversal

 \*/ class Solution { 

//traverse recursion 

```
 public List preorderTraversal(TreeNode root) { 
  ArrayList result = new ArrayList<>(); 
  traverse(root, result); 
  return result;
}
 
 public void traverse(TreeNode root, ArrayList<Integer> list){
    if (root == null){
       return;
    }
        
     list.add(root.val);
     traverse(root.left, list);
     traverse(root.right, list);
    }
}
 

```

} }

// non-recursion / interation way

```
   class Solution { //traverse recursion 
   public List preorderTraversal(TreeNode root) { 
   Stack<TreeNode> stack = new Stack<>(); 
   ArrayList<Integer> preorder = new ArrayList<>();
    
      if (root == null){
        return preorder;
    }
    
    stack.push(root);
    
    while(!stack.isEmpty()){
        TreeNode root = stack.pop();
        preorder.add(root.val);
        
        if(root.right != null){ //push right first because FILO
            stack.push(root.right);
        }
               
        if(root.left != null){
            stack.push(root.left);
        }
        
    }
    
   return preorder;
   }
}
```