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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?

题解：二叉树的先序排序

code：

package com.leetcode.code;import java.util.ArrayList;import java.util.List;class TreeNode {	     int val;	     TreeNode left;	     TreeNode right;	     TreeNode(int x) { val = x; }}public class Solution144 {		public void preorder(TreeNode root,List
   
     nodes)	{		if(root!= null)		{			nodes.add(root.val);			preorder(root.left, nodes);			preorder(root.right, nodes);		}	}		 public List
    
      preorderTraversal(TreeNode root) {	        	     List
     
       nodes = new ArrayList
      
       ();		   	     preorder(root, nodes);				 return nodes;	   }		public static void main(String[] args) {				TreeNode root = new TreeNode(1);		root.left=null;		root.right = new TreeNode(2);		root.right.left = new TreeNode(3);		root.right.right = null;		List
       
         nodes = new Solution144().preorderTraversal(root);	    		for(Integer n:nodes)		{			System.out.println(n);		}	}}
       
      
     
    
   

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