Flatten a binary tree in O(1) space and O(n) time complexity

FlattenTree.java

public class Program {
  
  internal class Node<T> {
    final T data;
    final Node<T> left;
    final Node<T> right;
    
    bool hasLeftChild() {
      return this.left != null;
    }
  }
  
  static void flattenTree(Node<int> root) {
    Node<int> currentRoot = root;
    Node<int> currentLeaf = root;
    
    while (currentRoot.hasLeftChild()) {
      while (currentLeaf.hasLeftChild()) {
        currentLeaf = currentLeaf.left;
      }
      
      currentLeaf.left = currentRoot.right;
      currentRoot.right = null;
      currentRoot = currentRoot.left;
    }
  }
}