Data Structure Binary Tree: Connect nodes at same level using constant extra space

http://www.geeksforgeeks.org/connect-nodes-at-same-level-with-o1-extra-space/

recursive:

 1 #include <iostream>

 2 #include <vector>

 3 #include <algorithm>

 4 #include <queue>

 5 #include <stack>

 6 #include <string>

 7 #include <fstream>

 8 using namespace std;

 9 

10 struct node {

11     int data;

12     struct node *left, *right, *next;

13     node() : data(0), left(NULL), right(NULL), next(NULL) { }

14     node(int d) : data(d), left(NULL), right(NULL), next(NULL) { }

15 };

16 

17 node *getnext(node *root) {

18     node *next = root->next;

19     while (next) {

20         if (next->left) return next->left;

21         if (next->right) return next->right;

22         next = next->next;

23     }

24     return NULL;

25 }

26 

27 void _connect(node *root) {

28     if (!root) return;

29     if (root->next) _connect(root->next);

30     if (root->left) {

31         if (root->right) {

32             root->left->next = root->right;

33             root->right->next = getnext(root);

34         }

35         else root->left->next = getnext(root);

36         _connect(root->left);

37     }

38     else if (root->right) {

39         root->right->next = getnext(root);

40         _connect(root->right);

41     }

42     else _connect(root->next);

43 }

44 

45 void connect(node *root) {

46     if (!root) return;

47     root->next = NULL;

48     _connect(root);

49 }

50 

51 int main() {

52     node *root = new node(10);

53     root->left = new node(8);

54     root->right = new node(2);

55     root->left->left = new node(3);

56     root->right->right = new node(90);

57     connect(root);

58     cout << root->data << "->" << (root->next? root->next->data : -1) << endl;

59     cout << root->left->data << "->" << (root->left->next? root->left->next->data : -1) << endl;

60     cout << root->right->data << "->" << (root->right->next? root->right->next->data : -1) << endl;

61     cout << root->left->left->data << "->" << (root->left->left->next? root->left->left->next->data : -1) << endl;

62     cout << root->right->right->data << "->" << (root->right->right->next? root->right->right->next->data : -1) << endl;

63     return 0;

64 }

iterative(better)

 1 #include <iostream>

 2 #include <vector>

 3 #include <algorithm>

 4 #include <queue>

 5 #include <stack>

 6 #include <string>

 7 #include <fstream>

 8 using namespace std;

 9 

10 struct node {

11     int data;

12     struct node *left, *right, *next;

13     node() : data(0), left(NULL), right(NULL), next(NULL) { }

14     node(int d) : data(d), left(NULL), right(NULL), next(NULL) { }

15 };

16 

17 node *getnext(node *root) {

18     node *next = root->next;

19     while (next) {

20         if (next->left) return next->left;

21         if (next->right) return next->right;

22         next = next->next;

23     }

24     return NULL;

25 }

26 

27 void connect(node *root) {

28     if (!root) return;

29     root->next = NULL;

30     while (root) {

31         node *p = root;

32         while (p) {

33             if (p->left) {

34                 if (p->right) p->left->next = p->right;

35                 else p->left->next = getnext(p);

36             }

37             if (p->right) p->right->next = getnext(p);

38             p = p->next;

39         }

40         if (root->left) root = root->left;

41         else if (root->right) root = root->right;

42         else root = root->next;

43     }

44 }

45 

46 int main() {

47     node *root = new node(10);

48     root->left = new node(8);

49     root->right = new node(2);

50     root->left->left = new node(3);

51     root->right->right = new node(90);

52     connect(root);

53     cout << root->data << "->" << (root->next? root->next->data : -1) << endl;

54     cout << root->left->data << "->" << (root->left->next? root->left->next->data : -1) << endl;

55     cout << root->right->data << "->" << (root->right->next? root->right->next->data : -1) << endl;

56     cout << root->left->left->data << "->" << (root->left->left->next? root->left->left->next->data : -1) << endl;

57     cout << root->right->right->data << "->" << (root->right->right->next? root->right->right->next->data : -1) << endl;

58     return 0;

59 }