LeetCode—binary-tree-preorder-traversal（二叉树的前序遍历）—Java

题目描述：

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree{1,#,2,3},

   1
    \
     2
    /
   3

思路解析：

需要一个用于递归前序遍历的helper（void）：

• 先存入ArrayList<Integer> re = new ArrayList<Integer>;
• 然后遍历左子树，再遍历右子树

代码：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> re = new ArrayList<Integer>();
        if(root ==null)
            return re;
        helper(re,root);
        return re;
    }
    public void helper(ArrayList<Integer> re,TreeNode root){
        if(root==null)
            return;
        re.add(root.val);
        helper(re,root.left);
        helper(re,root.right);
    }
}