LintCode: Binary Tree Preorder Traversal

Problem:


Idea:

The trick here is to set a private variable result and refresh it in the helper method, then do the preorder traversal in a recursive way.

Code:

/**
* 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 {
/**
* @param root: The root of binary tree.
* @return: Preorder in ArrayList which contains node values.
*/
private ArrayList<Integer> result = new ArrayList<Integer>();
public ArrayList<Integer> preorderTraversal(TreeNode root) {
// write your code here
// handle corner case
if (root == null) {
return result;
}
helper(root);
return result;
}
private void helper(TreeNode node) {
if (node == null) {
return;
}
// refresh result
result.add(node.val);
// go for left and right
helper(node.left);
helper(node.right);
}
}