LintCode | 66. 二叉樹的前序遍歷

給出一棵二叉樹，返回其節點值的前序遍歷。
題目鏈接

/**
* 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> list = new ArrayList<Integer>();

    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        if(root == null) return list;
        list.add(root.val);
        preorderTraversal(root.left);
        preorderTraversal(root.right);
        return list;
    }
}