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;    }}