Binary Tree Preorder Traversal

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

Have you met this question in a real interview? Yes

Example

Given:

1

/ \

2 3

/ \

4 5

return [1,2,4,5,3].

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/
public class Solution {
public ArrayList<Integer> preorderTraversal(TreeNode root) {
ArrayList<Integer> result = new ArrayList<Integer>();
// null or leaf
if (root == null) {
return result;
}

// Divide
ArrayList<Integer> left = preorderTraversal(root.left);
ArrayList<Integer> right = preorderTraversal(root.right);

// Conquer
result.add(root.val);
result.addAll(left);
result.addAll(right);
return result;
}
}