Convert a given Binary Tree to Doubly Linked List

The question and solution are from: http://www.geeksforgeeks.org/convert-given-binary-tree-doubly-linked-list-set-3/

Given a Binary Tree (BT), convert it to a Doubly Linked List(DLL) In-Place. The left and right pointers in nodes are to be used as previous and next pointers respectively in converted DLL. The order of nodes in DLL must be same as Inorder of the given Binary Tree. The first node of Inorder traversal (left most node in BT) must be head node of the DLL.

曾经的我能够想到的方法就是利用LinkedList来保存每个node，然后修改每个node的left和right。

下面这个方法现在想起来好像并不那么复杂了， 值得学习一下。

这种方法使用inorder traversal, 他先一直traverse到最左边，然后set head. 在traverse右边之前，把当前root设置成prev, 这样就可以把prev和后面部分的dll连起来。

 public class BinaryTree {
     Node head, prev = null;
     void binaryTree2DoubleLinkedList(Node root) {
         if (root == null) return;
         binaryTree2DoubleLinkedList(root.left);
         if (prev == null) {
             head = root;
         } else {
             root.left = prev;
             prev.right = root;
         }
         prev = root;
         binaryTree2DoubleLinkedList(root.right);
     }

     void printList(Node node) {
         while (node != null) {
             System.out.print(node.data + " ");
             node = node.right;
         }
     }
     public static void main(String[] args) {
         BinaryTree tree = new BinaryTree();
         Node root = new Node();
         root.left = new Node();
         root.right = new Node();
         root.left.left = new Node();
         root.left.right = new Node();
         root.right.left = new Node();

         tree.binaryTree2DoubleLinkedList(root);
         tree.printList(tree.head);

     }

     public Node binaryTreeToDDL(Node root) {
         if (root == null) return null;

         Node leftHead = binaryTreeToDDL(root.left);
         Node rightHead = binaryTreeToDDL(root.right);

         Node newHead = null;

         if (leftHead == null) {
             newHead = root;
         } else {
             Node leftEnd = leftHead;
             while (leftEnd.right != null) {
                 leftEnd = leftEnd.right;
             }
             leftEnd.right = root;
             root.left = leftEnd;
             newHead = leftHead;
         }

         if (rightHead != null) {
             rightHead.left = root;
             root.right = rightHead;
         }
         return newHead;
     }
 }

 class Node {
     int data;
     Node left, right;

     public Node(int data) {
         this.data = data;
         left = right = null;
     }
 }