题目

描述

给出一棵二叉树，返回其节点值的前序遍历。

样例

给出一棵二叉树 {1,#,2,3},
1 \ 2 / 3
返回 [1,2,3].

解答

思路

前序遍历二叉树

代码

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> a = new ArrayList<Integer>();
        preOrder(a, root);
        return a;
    }
    
    private void preOrder(ArrayList<Integer> a, TreeNode root){
        if(root == null) return;
        a.add(root.val);
        preOrder(a, root.left);
        preOrder(a, root.right);
    }
}