leetcode------Binary Tree Preorder Traversal

标题：	Binary Tree Preorder Traversal
通过率：	36.2%
难度：	中等

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].

Note: Recursive solution is trivial, could you do it iteratively?

二叉树的前序遍历，直接进行递归访问即可，代码如下：

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