# Binary Tree Preorder Traversal Problem

## Description

LeetCode Problem 144.

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

Example 1:

``````1
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Input: root = [1,null,2,3]
Output: [1,2,3]
``````

Example 2:

``````1
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Input: root = []
Output: []
``````

Example 3:

``````1
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Input: root = [1]
Output: [1]
``````

Example 4:

``````1
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Input: root = [1,2]
Output: [1,2]
``````

Example 5:

``````1
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Input: root = [1,null,2]
Output: [1,2]
``````

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

## Sample C++ Code

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class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> preorder;

if (root == NULL)
return preorder;

stack<TreeNode*> s;
s.push(root);
TreeNode* tp;

while (!s.empty()) {
tp = s.top();
s.pop();
if (tp == NULL)
continue;
s.push(tp->right);
s.push(tp->left);
preorder.push_back(tp->val);
}
return preorder;
}
};
``````