English
144. Binary Tree Preorder Traversal 
Problem Statement:
Given the root of a binary tree, return the preorder traversal of its nodes' values.
Example 1:
Input: root = [1,null,2,3]
Output: [1,2,3]
Example 2:
Input: root = []
Output: []
Example 3:
Input: root = [1]
Output: [1]
Constraints:
The number of nodes in the tree is in the range [0, 100]. -100 <= Node.val <= 100
Follow up: Recursive solution is trivial, could you do it iteratively?
Solution:
java
List<Integer> list = new ArrayList<Integer>();
// Iterative Approach
public List<Integer> preorderTraversal(TreeNode root) {
if (root == null)
return list;
Stack<TreeNode> stack = new Stack<>();
stack.push(root);
while (!stack.isEmpty()) {
TreeNode current = stack.pop();
list.add(current.val);
if (current.right != null)
stack.push(current.right);
if (current.left != null)
stack.push(current.left);
}
return list;
}
// Recursive Approach
public List<Integer> preorderTraversalRecursive(TreeNode root) {
if (root != null) {
list.add(root.val);
preorderTraversalRecursive(root.left);
preorderTraversalRecursive(root.right);
}
return list;
}
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