feat: Add Morris In-Order Traversal to BinaryTreeTraversal

pydatastructs/trees/_backend/cpp/BinaryTreeTraversal.hpp

@@ -171,6 +171,47 @@ static PyObject* BinaryTreeTraversal__out_order(BinaryTreeTraversal* self, PyObj
     return visit;
 }
 
+static PyObject* BinaryTreeTraversal_morris_in_order_traversal(BinaryTreeTraversal* self, PyObject *args) {
+    PyObject* node = PyObject_GetItem(args, PyZero);
+    if (node == Py_None) {
+        node = self->tree->root_idx;
+    }
+
+    PyObject* traversal = PyList_New(0);
+    ArrayForTrees* tree = self->tree->tree;
+    long current = PyLong_AsLong(node);
+
+    while (current != Py_None) {
+        TreeNode* current_node = reinterpret_cast<TreeNode*>(tree->_one_dimensional_array->_data[current]);
+        if (current_node->left == Py_None) {
+            // If there's no left child, visit the current node
+            PyList_Append(traversal, reinterpret_cast<PyObject*>(current_node));
+            current = PyLong_AsLong(current_node->right);
+        } else {
+            // Find the in-order predecessor (rightmost node in the left subtree)
+            long predecessor = PyLong_AsLong(current_node->left);
+            TreeNode* predecessor_node = reinterpret_cast<TreeNode*>(tree->_one_dimensional_array->_data[predecessor]);
+            while (predecessor_node->right != Py_None && predecessor_node->right != node) {
+                predecessor = PyLong_AsLong(predecessor_node->right);
+                predecessor_node = reinterpret_cast<TreeNode*>(tree->_one_dimensional_array->_data[predecessor]);
+            }
+
+            if (predecessor_node->right == Py_None) {
+                // Make the current node the right child of the predecessor
+                predecessor_node->right = node;
+                current = PyLong_AsLong(current_node->left);
+            } else {
+                // Revert the changes made to the tree
+                predecessor_node->right = Py_None;
+                PyList_Append(traversal, reinterpret_cast<PyObject*>(current_node));
+                current = PyLong_AsLong(current_node->right);
+            }
+        }
+    }
+
+    return traversal;
+}
+
 static PyObject* BinaryTreeTraversal_depth_first_search(BinaryTreeTraversal* self, PyObject *args, PyObject *kwds) {
     Py_INCREF(Py_None);
     PyObject* node = Py_None;
@@ -242,12 +283,12 @@ static struct PyMethodDef BinaryTreeTraversal_PyMethodDef[] = {
     {"_in_order", (PyCFunction) BinaryTreeTraversal__in_order, METH_VARARGS, NULL},
     {"_out_order", (PyCFunction) BinaryTreeTraversal__out_order, METH_VARARGS, NULL},
     {"_post_order", (PyCFunction) BinaryTreeTraversal__post_order, METH_VARARGS, NULL},
+    {"morris_in_order_traversal", (PyCFunction) BinaryTreeTraversal_morris_in_order_traversal, METH_VARARGS, NULL},
     {"depth_first_search", (PyCFunction) BinaryTreeTraversal_depth_first_search, METH_VARARGS | METH_KEYWORDS, NULL},
     {"breadth_first_search", (PyCFunction) BinaryTreeTraversal_breadth_first_search, METH_VARARGS | METH_KEYWORDS, NULL},
     {NULL}
 };
 
-
 static PyTypeObject BinaryTreeTraversalType = {
     /* tp_name */ PyVarObject_HEAD_INIT(NULL, 0) "BinaryTreeTraversal",
     /* tp_basicsize */ sizeof(BinaryTreeTraversal),

pydatastructs/trees/binary_trees.py

@@ -1761,6 +1761,50 @@ def breadth_first_search(self, node=None, strategy='queue'):
                 q.append(tree[node].right)
         return visit
 
+    def morris_in_order_traversal(self, node=None):
+        """
+        Perform in-order traversal using Morris Traversal.
+
+        Parameters
+        ==========
+
+        node : int
+            The index of the node from where the traversal has to be instantiated.
+            By default, set to, root index.
+
+        Returns
+        =======
+
+        list
+            Each element of the list is of type `TreeNode`.
+        """
+        if node is None:
+            node = self.tree.root_idx
+
+        traversal = []
+        current = node
+
+        while current is not None:
+            if self.tree.tree[current].left is None:
+                traversal.append(self.tree.tree[current])
+                current = self.tree.tree[current].right
+            else:
+                predecessor = self.tree.tree[current].left
+                while (
+                    self.tree.tree[predecessor].right is not None
+                    and self.tree.tree[predecessor].right != current
+                ):
+                    predecessor = self.tree.tree[predecessor].right
+
+                if self.tree.tree[predecessor].right is None:
+                    self.tree.tree[predecessor].right = current
+                    current = self.tree.tree[current].left
+                else:
+                    self.tree.tree[predecessor].right = None
+                    traversal.append(self.tree.tree[current])
+                    current = self.tree.tree[current].right
+        return traversal
+
 class BinaryIndexedTree(object):
     """
     Represents binary indexed trees

pydatastructs/trees/tests/test_binary_trees.py

@@ -151,6 +151,9 @@ def _test_BinaryTreeTraversal(backend):
     bfs = trav.breadth_first_search()
     assert [node.key for node in bfs] == ['F', 'B', 'G', 'A', 'D', 'I', 'C', 'E', 'H']
 
+    morris_in_order = trav.morris_in_order_traversal()
+    assert [node.key for node in morris_in_order] == ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I']
+
     assert raises(NotImplementedError, lambda: trav.breadth_first_search(strategy='iddfs'))
     assert raises(NotImplementedError, lambda: trav.depth_first_search(order='in_out_order'))
     assert raises(TypeError, lambda: BTT(1))