题解 | 求二叉树的前序遍历

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList
     */
    public ArrayList<Integer> preorderTraversal (TreeNode root) {
        // write code here
        ArrayList<Integer> result=new ArrayList();

        preorderHelper(root,result);

        return result;

    }

    public void preorderHelper(TreeNode node,ArrayList<Integer> result){
        if(null==node){
            return;
        }

        result.add(node.val);
        preorderHelper(node.left,result);
        preorderHelper(node.right,result);
    }
}