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> inorderTraversal(TreeNode* root) {
  
}
};

Given the root of a binary tree, return the inorder traversal of its nodes' values.

Example 1:

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

Example 2:

Input: root = []
Output: []

Example 3:

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

Example 4:

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

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

The code is tested and should run as a single file in any IDE. The main method is also included. An output is also attached. The idea of inorder traversal is simple:

• traverse left subtree
• process root
• traverse right subtree

Helpful and self-explanatory comments are added to the code.

Code

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 Solution {
  public static void rec(TreeNode root, List <Integer> result) {
    if (root == null) {
      // If root is null, just return
      return;
    }
    // Visit left subtree
    rec(root.left, result);
    // Visit root
    result.add(root.val);
    // Visit right subtree
    rec(root.right, result);
  }
  public static List<Integer> inorderTraversal(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       5
    3   4   6   7
    inorder - [3, 2, 4, 1, 6, 5, 7]
    */
    TreeNode root = new TreeNode(1);
    root.left = new TreeNode(2);
    root.left.left = new TreeNode(3);
    root.left.right = new TreeNode(4);
    root.right = new TreeNode(5);
    root.right.left = new TreeNode(6);
    root.right.right = new TreeNode(7);
    List<Integer> result = inorderTraversal(root);
    System.out.print("Inorder: "); 
    for (int i = 0; i < result.size(); i++) { 
      System.out.print(result.get(i) + " "); 
    }
  }
}

Output