Binary Tree Preorder Traversal

Binary Tree Preorder Traversal

 

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].

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    List<Integer> result = new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        if(null == root)
            return result;
        else{
            result.add(new Integer(root.val));
            preorderTraversal(root.left);
            preorderTraversal(root.right);
            return result;
        }
    }
}