Data Structure Binary Tree: Convert a given Binary Tree to Doubly Linked List

http://www.geeksforgeeks.org/in-place-convert-a-given-binary-tree-to-doubly-linked-list/

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 #include <map>
 using namespace std;

 struct node {
     int data;
     struct node *left, *right;
     node() : data(), left(NULL), right(NULL) { }
     node(int d) : data(d), left(NULL), right(NULL) { }
 };

 node *_convert(node *root) {
     if (!root) return NULL;
     if (root->left) {
         node *left = _convert(root->left);
         while (left->right) left = left->right;
         left->right = root;
         root->left = left;
     }
     if (root->right) {
         node *right = _convert(root->right);
         while (right->left) right = right->left;
         right->left = root;
         root->right = right;
     }
     return root;
 }

 node *convert(node *root) {
     if (!root) return NULL;
     root = _convert(root);
     while (root->left) root = root->left;
     return root;
 }

 int main() {
     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();
     node *head = convert(root);
     while (head) {
         cout << head->data << " ";
         head = head->right;
     }
     return ;
 }

http://www.geeksforgeeks.org/convert-a-given-binary-tree-to-doubly-linked-list-set-2/

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 #include <map>
 using namespace std;

 struct node {
     int data;
     struct node *left, *right;
     node() : data(), left(NULL), right(NULL) { }
     node(int d) : data(d), left(NULL), right(NULL) { }
 };

 void fixpre(node *root) {
     static node *pre = NULL;
     if (!root) return;
     fixpre(root->left);
     root->left = pre;
     pre = root;
     fixpre(root->right);
 }

 node *fixnext(node *root) {
     node *pre = NULL;
     while (root && root->right) root = root->right;
     while (root && root->left) {
         pre = root;
         root = root->left;
         root->right = pre;
     }
     return root;
 }

 node *convert(node *root) {
     if (!root) return NULL;
     fixpre(root);
     return fixnext(root);
 }

 int main() {
     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();
     node *head = convert(root);
     while (head) {
         cout << head->data << " ";
         head = head->right;
     }
     return ;
 }

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

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 #include <map>
 using namespace std;

 struct node {
     int data;
     struct node *left, *right;
     node() : data(), left(NULL), right(NULL) { }
     node(int d) : data(d), left(NULL), right(NULL) { }
 };

 void convert(node *root, node *&head) {
     if (!root) return;
     static node *pre = NULL;
     convert(root->left, head);
     if (pre == NULL) head = root;
     else {
         root->left = pre;
         pre->right = root;
     }
     pre = root;
     convert(root->right, head);
 }

 int main() {
     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();
     node *head = NULL;
     convert(root, head);
     while (head) {
         cout << head->data << " ";
         head = head->right;
     }
     return ;
 }