Merge pull request #134 from PrakashShukla0907/add-flatten-binary-tree

Added Flatten Binary Tree algorithm in C++ under CPP/tree/

Author: admirerr

CPP/tree/flatten_binary_tree.cpp

@@ -0,0 +1,81 @@
+#include <iostream>
+using namespace std;
+#include <stack>
+// Definition for a binary tree node.
+struct Node
+{
+ int data;
+ Node *left;
+ Node *right;
+ Node(int val)
+ {
+  data = val;
+  left = NULL;
+  right = NULL;
+ }
+};
+
+//build binary tree from user input
+// Input format: Enter node values in pre-order fashion. Use -1 to indicate NULL nodes
+Node *buildTree()
+{
+ int val;
+ cin >> val;
+ if (val == -1)
+  return NULL;
+ Node *root = new Node(val);
+ root->left = buildTree();
+ root->right = buildTree();
+ return root;
+}
+
+Node *flatten(Node *root)  // flatten the binary tree to a linked list in-place time: O(n) space: O(1)
+{
+ if (!root)
+  return NULL;
+ Node *curr = root;
+
+ while (curr != NULL)
+ {
+  if (curr->left != NULL)
+  {
+   Node *prev = curr->left;
+   while (prev->right)
+   {
+    prev = prev->right;
+   }
+   prev->right = curr->right;
+   curr->right = curr->left;
+   curr->left = NULL;
+  }
+  curr = curr->right;
+ }
+ return root;
+}
+
+void inorder(Node *root)  // inorder traversal to display the tree
+{
+ if (!root)
+  return;
+ inorder(root->left);
+ cout << root->data << " ";
+ inorder(root->right);
+}
+
+int main()
+{
+ cout << "Build the binary tree (use -1 for NULL nodes): ";
+ Node *root = buildTree(); // Example input: 1 2 4 -1 -1 5 -1 -1 3 -1 -1
+
+ cout << "Inorder Traversal of original tree: ";
+ inorder(root);
+ cout << endl;
+
+ root = flatten(root);
+
+ cout << "Inorder Traversal of flattened tree: ";
+ inorder(root);
+ cout << endl;
+
+ return 0;
+}