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Description
tab: English
<p>Given the <code>root</code> of a binary tree, return <em>the preorder traversal of its nodes' values</em>.</p>
<p> </p>
<p><strong class="example">Example 1:</strong></p>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">root = [1,null,2,3]</span></p>
<p><strong>Output:</strong> <span class="example-io">[1,2,3]</span></p>
<p><strong>Explanation:</strong></p>
<p><img alt="" src="https://assets.leetcode.com/uploads/2024/08/29/screenshot-2024-08-29-202743.png" style="width: 200px; height: 264px;" /></p>
</div>
<p><strong class="example">Example 2:</strong></p>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">root = [1,2,3,4,5,null,8,null,null,6,7,9]</span></p>
<p><strong>Output:</strong> <span class="example-io">[1,2,4,5,6,7,3,8,9]</span></p>
<p><strong>Explanation:</strong></p>
<p><img alt="" src="https://assets.leetcode.com/uploads/2024/08/29/tree_2.png" style="width: 350px; height: 286px;" /></p>
</div>
<p><strong class="example">Example 3:</strong></p>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">root = []</span></p>
<p><strong>Output:</strong> <span class="example-io">[]</span></p>
</div>
<p><strong class="example">Example 4:</strong></p>
<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">root = [1]</span></p>
<p><strong>Output:</strong> <span class="example-io">[1]</span></p>
</div>
<p> </p>
<p><strong>Constraints:</strong></p>
<ul>
<li>The number of nodes in the tree is in the range <code>[0, 100]</code>.</li>
<li><code>-100 <= Node.val <= 100</code></li>
</ul>
<p> </p>
<p><strong>Follow up:</strong> Recursive solution is trivial, could you do it iteratively?</p>
---
[submissions](https://leetcode.com/problems/binary-tree-preorder-traversal/submissions/) | [solutions](https://leetcode.com/problems/binary-tree-preorder-traversal/solutions/)
tab: 中文
<p>给你二叉树的根节点 <code>root</code> ，返回它节点值的 <strong>前序</strong><em> </em>遍历。</p>
<p> </p>
<p><strong class="example">示例 1：</strong></p>
<div class="example-block">
<p><strong>输入：</strong><span class="example-io">root = [1,null,2,3]</span></p>
<p><strong>输出：</strong><span class="example-io">[1,2,3]</span></p>
<p><strong>解释：</strong></p>
<p><img alt="" src="https://assets.leetcode.com/uploads/2024/08/29/screenshot-2024-08-29-202743.png" style="width: 200px; height: 264px;" /></p>
</div>
<p><strong class="example">示例 2：</strong></p>
<div class="example-block">
<p><span class="example-io"><b>输入：</b>root = [1,2,3,4,5,null,8,null,null,6,7,9]</span></p>
<p><span class="example-io"><b>输出：</b>[1,2,4,5,6,7,3,8,9]</span></p>
<p><strong>解释：</strong></p>
<p><img alt="" src="https://assets.leetcode.com/uploads/2024/08/29/tree_2.png" style="width: 350px; height: 286px;" /></p>
</div>
<p><strong class="example">示例 3：</strong></p>
<div class="example-block">
<p><span class="example-io"><b>输入：</b>root = []</span></p>
<p><span class="example-io"><b>输出：</b>[]</span></p>
</div>
<p><strong class="example">示例 4：</strong></p>
<div class="example-block">
<p><strong>输入：</strong><span class="example-io">root = [1]</span></p>
<p><span class="example-io"><b>输出：</b>[1]</span></p>
</div>
<p> </p>
<p><strong>提示：</strong></p>
<ul>
<li>树中节点数目在范围 <code>[0, 100]</code> 内</li>
<li><code>-100 <= Node.val <= 100</code></li>
</ul>
<p> </p>
<p><strong>进阶：</strong>递归算法很简单，你可以通过迭代算法完成吗？</p>
---
[提交记录](https://leetcode.cn/problems/binary-tree-preorder-traversal/submissions/) | [题解](https://leetcode.cn/problems/binary-tree-preorder-traversal/solution/)
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