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].
思路：二叉树的前序遍历

package leetcode;
import java.util.ArrayList;
import java.util.List;

public class BinaryTreePreorderTraversal {
	public List<Integer> preorderTraversal(TreeNode root) {
		List<Integer> list = new ArrayList<Integer>();
		if(root == null) return list;
		list.add(root.val);
		if(root.left != null) list.addAll(preorderTraversal(root.left));
		if(root.right != null) list.addAll(preorderTraversal(root.right));
        return list;
    }
	
}