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本題知識點： 樹 leetcode
演算法知識視訊講解
題目描述

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

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

1

2
/
3

return[1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

思路：https://blog.csdn.net/ccnuacmhdu/article/details/85227092

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    private ArrayList<Integer> list = new ArrayList<Integer>();
    public ArrayList<
 
Integer> preorderTraversal(TreeNode root) {
        //這道題明顯類似於上一題後序遍歷的解法，後序遍歷用棧:
        //https://blog.csdn.net/ccnuacmhdu/article/details/85227092
        //這道題依然是用棧！！細想下，不能用佇列！！遞迴與棧本一家啊，兩道非遞迴遍歷題感觸深
        if(root == null){
            return list;
        }
        ArrayDeque<TreeNode> stack = new ArrayDeque
 
<TreeNode>();
        stack.addFirst(root);
        while(stack.isEmpty() == false){
            TreeNode node = stack.removeFirst();
            list.add(node.val);
            if(node.right != null){
                stack.addFirst(node.right);
            }
            if(node.left != null){
                stack.addFirst(node.left);
            }
        }
        return list;
    }
}