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].

用递归算法还是比较简单的：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    List<Integer> re = new ArrayList<Integer>();
    //先序遍历二叉树
    public void PSF(TreeNode N) {
        if(N!=null) {
            re.add(N.val);
            if(N.left!=null) PSF(N.left);
            if(N.right!=null) PSF(N.right);
        }
    }
    public List<Integer> preorderTraversal(TreeNode root) {
        PSF(root);
        return re;
    }
}