# 144 Binary Tree Preorder Traversal – Medium

### Problem:

Given a binary tree, return the preorder traversal of its nodes’ values.

For example: Given binary tree {1,#,2,3},

1 \ 2 / 3 return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

### Thoughts:

It is trivial to use recursion approach to solve this problem.

Preorder is to do a root, left, right.

Always append when meets a root. Then go deeper to the left subtree if got one.

Using a Stack is very helpful, which guaranteed all elements in the subtree will be appended prior to the right child.

### Solutions:

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
Stack<TreeNode> s = new Stack<TreeNode>();
if (root == null) {
return result;
}
s.push(root);
while (s.size() > 0) {
TreeNode node = s.pop();
if (node.right != null) {
s.push(node.right);
}
if (node.left != null) {
s.push(node.left);
}
}
return result;
}

}
``````