把二叉树转换成双向链表

转字 https://www.geeksforgeeks.org/convert-given-binary-tree-doubly-linked-list-set-3/

Convert a given Binary Tree to 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.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

 

// A C++ program for in-place conversion of Binary Tree to DLL #include <iostream> using namespace std; /* A binary tree node has data, and left and right pointers */ struct node {     int data;     node* left;     node* right; }; // A simple recursive function to convert a given Binary tree to Doubly // Linked List // root --> Root of Binary Tree // head --> Pointer to head node of created doubly linked list void BinaryTree2DoubleLinkedList(node *root, node **head) {     // Base case     if (root == NULL) return;     // Initialize previously visited node as NULL. This is     // static so that the same value is accessible in all recursive     // calls     static node* prev = NULL;     // Recursively convert left subtree     BinaryTree2DoubleLinkedList(root->left, head);     // Now convert this node     if (prev == NULL)         *head = root;     else     {         root->left = prev;         prev->right = root;     }     prev = root;     // Finally convert right subtree     BinaryTree2DoubleLinkedList(root->right, head); } /* Helper function that allocates a new node with the    given data and NULL left and right pointers. */ node* newNode(int data) {     node* new_node = new node;     new_node->data = data;     new_node->left = new_node->right = NULL;     return (new_node); } /* Function to print nodes in a given doubly linked list */ void printList(node *node) {     while (node!=NULL)     {         cout << node->data << " ";         node = node->right;     } } /* Driver program to test above functions*/ int main() {     // Let us create the tree shown in above diagram     node *root        = newNode(10);     root->left        = newNode(12);     root->right       = newNode(15);     root->left->left  = newNode(25);     root->left->right = newNode(30);     root->right->left = newNode(36);     // Convert to DLL     node *head = NULL;     BinaryTree2DoubleLinkedList(root, &head);     // Print the converted list     printList(head);     return 0; }

Run on IDE
Output:

25 12 30 10 36 15

Note that use of static variables like above is not a recommended practice (we have used static for simplicity). Imagine a situation where same function is called for two or more trees, the old value of prev would be used in next call for a different tree. To avoid such problems, we can use double pointer or reference to a pointer.

Time Complexity: The above program does a simpl