Question

Code using JAVA:

must include "Main Method" for IDE testing!

/**
* 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) {
  
}
};

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]

Example 4:

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

Example 5:

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

Answer 1

import java.util.*;

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;
}
}

public class Main {
public static void rec(TreeNode root, List <Integer> result) {
if (root == null) {
// If root is null, just return
return;
}
// Visit root
result.add(root.val);
// Visit left subtree
rec(root.left, result);
// Visit right subtree
rec(root.right, result);
}
public static List<Integer> preorderTraversal(TreeNode root) {
// Array to store the result
List<Integer> result = new ArrayList<Integer> ();
// Call recursive inorder traversal function
rec(root, result);
// Return result
return result;
}
public static void main(String []args) {
/*
1
2 3
4 5
preorder - [3, 2, 4, 1, 6, 5, 7]
*/
TreeNode root = new TreeNode(1);
root.left = new TreeNode(2);
root.left.left = new TreeNode(4);
root.left.right = new TreeNode(5);
root.right = new TreeNode(3);
List<Integer> result = preorderTraversal(root);
System.out.print("preorder: ");
for (int i = 0; i < result.size(); i++) {
System.out.print(result.get(i) + " ");
}
}
}