Leetcode144——Binary Tree Preorder Traversal

文章作者：Tyan
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1. 问题描述

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

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

   1\      2/3

return [1,2,3].

2. 求解

这个题就是一个树的先序遍历问题，最简单的方案就是递归的遍历子树，要注意递归退出的条件。

/*** Definition for a binary tree node.* public class TreeNode {* int val;* TreeNode left;* TreeNode right;* TreeNode(int x) { val = x; }* }*/
public class Solution {public List<Integer> preorderTraversal(TreeNode root) {List<Integer> list = new ArrayList<Integer>();if(root == null) {return list;}list.add(root.val);List<Integer> left = preorderTraversal(root.left);List<Integer> right = preorderTraversal(root.right);list.addAll(left);list.addAll(right);return list;}
}