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+/**%0A * Problem Link: https://leetcode.com/problems/binary-tree-preorder-traversal/%0A * %0A * Iteratively traverse all the tree nodes.%0A * %0A * Definition for a binary tree node.%0A * public class TreeNode %7B%0A *     int val;%0A *     TreeNode left;%0A *     TreeNode right;%0A *     TreeNode(int x) %7B val = x; %7D%0A * %7D%0A */%0Apublic class Solution %7B%0A    public List%3CInteger%3E preorderTraversal(TreeNode root) %7B%0A        Stack%3CTreeNode%3E stack = new Stack%3CTreeNode%3E();%0A        List%3CInteger%3E list = new ArrayList%3CInteger%3E();%0A        if(root == null) return list;%0A        stack.push(root);%0A        while(!stack.empty()) %7B%0A            TreeNode node = stack.pop();%0A            list.add(node.val);%0A            if(node.right != null) stack.push(node.right);%0A            if(node.left != null) stack.push(node.left);%0A        %7D%0A        return list;%0A    %7D%0A%7D%0A

Create Binary_Tree_Preorder_Traversal.java