144. Binary Tree Preorder Traversal LeetCode Solution

In this guide, you will get 144. Binary Tree Preorder Traversal LeetCode Solution with the best time and space complexity. The solution to Binary Tree Preorder Traversal problem is provided in various programming languages like C++, Java, and Python. This will be helpful for you if you are preparing for placements, hackathons, interviews, or practice purposes. The solutions provided here are very easy to follow and include detailed explanations.

Problem Statement
Complexity Analysis
Binary Tree Preorder Traversal solution in C++
Binary Tree Preorder Traversal solution in Java
Binary Tree Preorder Traversal solution in Python
Additional Resources

144. Binary Tree Preorder Traversal LeetCode Solution image

Problem Statement of Binary Tree Preorder Traversal

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

Example 1:

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

Example 2:

Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
Output: [1,2,4,5,6,7,3,8,9]
Explanation:

Example 3:

Input: root = []
Output: []

Example 4:

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

Constraints:

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

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

Complexity Analysis

Time Complexity: O(n)
Space Complexity: O(h)

144. Binary Tree Preorder Traversal LeetCode Solution in C++

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

 private:
  void preorder(TreeNode* root, vector<int>& ans) {
    if (root == nullptr)
      return;

    ans.push_back(root->val);
    preorder(root->left, ans);
    preorder(root->right, ans);
  }
};
/* code provided by PROGIEZ */

144. Binary Tree Preorder Traversal LeetCode Solution in Java

class Solution {
  public List<Integer> preorderTraversal(TreeNode root) {
    List<Integer> ans = new ArrayList<>();
    preorder(root, ans);
    return ans;
  }

  private void preorder(TreeNode root, List<Integer> ans) {
    if (root == null)
      return;

    ans.add(root.val);
    preorder(root.left, ans);
    preorder(root.right, ans);
  }
}
// code provided by PROGIEZ

144. Binary Tree Preorder Traversal LeetCode Solution in Python

class Solution:
  def preorderTraversal(self, root: TreeNode | None) -> list[int]:
    ans = []

    def preorder(root: TreeNode | None) -> None:
      if not root:
        return

      ans.append(root.val)
      preorder(root.left)
      preorder(root.right)

    preorder(root)
    return ans
 # code by PROGIEZ