In this post, we are going to solve the 144. Binary Tree Preorder Traversal problem of Leetcode. This problem 144. Binary Tree Preorder Traversal is a Leetcode easy level problem. Let’s see the code, 144. Binary Tree Preorder Traversal – Leetcode Solution.

Problem

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]

Example 2 :

Input: root = []
Output: []

Example 3 :

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

Constraints

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

Now, let’s see the code of 144. Binary Tree Preorder Traversal – Leetcode Solution.

Binary Tree Preorder Traversal – Leetcode Solution

144. Binary Tree Preorder Traversal – Solution in Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    public void preOrder(TreeNode root, List<Integer> ans){
        if(root == null) return;
        
        ans.add(root.val);
        preOrder(root.left,ans);
        preOrder(root.right,ans);
    }
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<Integer>();
        preOrder(root,ans);
        return ans;
    }
}

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

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> v;
        preTraversal(root, v);
        return v;
    }
    void preTraversal(TreeNode* root, vector<int>& v){
        if(!root) return;
        v.push_back(root->val);
        preTraversal(root->left, v);
        preTraversal(root->right, v);
    }
};

144. Binary Tree Preorder Traversal – Solution in Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def preorderTraversal(self, root):
        res = []
        self.dfs(root, res)
        return res
    
    def dfs(self, root, res):
        if root:
            res.append(root.val)
            self.dfs(root.left, res)
            self.dfs(root.right, res)
        

Note: This problem 144. Binary Tree Preorder Traversal is generated by Leetcode but the solution is provided by CodingBroz. This tutorial is only for Educational and Learning purpose.