Added code to merge two trees (#4121)

* Added code to merge two trees

* Added doctest and type hints

* Added pre-commit

data_structures/binary_tree/merge_two_binary_trees.py

@@ -0,0 +1,93 @@
+#!/usr/local/bin/python3
+"""
+Problem Description: Given two binary tree, return the merged tree.
+The rule for merging is that if two nodes overlap, then put the value sum of
+both nodes to the new value of the merged node. Otherwise, the NOT null node
+will be used as the node of new tree.
+"""
+from typing import Optional
+
+
+class Node:
+    """
+    A binary node has value variable and pointers to its left and right node.
+    """
+
+    def __init__(self, value: int = 0) -> None:
+        self.value = value
+        self.left: Optional[Node] = None
+        self.right: Optional[Node] = None
+
+
+def merge_two_binary_trees(tree1: Optional[Node], tree2: Optional[Node]) -> Node:
+    """
+    Returns root node of the merged tree.
+
+    >>> tree1 = Node(5)
+    >>> tree1.left = Node(6)
+    >>> tree1.right = Node(7)
+    >>> tree1.left.left = Node(2)
+    >>> tree2 = Node(4)
+    >>> tree2.left = Node(5)
+    >>> tree2.right = Node(8)
+    >>> tree2.left.right = Node(1)
+    >>> tree2.right.right = Node(4)
+    >>> merged_tree = merge_two_binary_trees(tree1, tree2)
+    >>> print_preorder(merged_tree)
+    9
+    11
+    2
+    1
+    15
+    4
+    """
+    if tree1 is None:
+        return tree2
+    if tree2 is None:
+        return tree1
+
+    tree1.value = tree1.value + tree2.value
+    tree1.left = merge_two_binary_trees(tree1.left, tree2.left)
+    tree1.right = merge_two_binary_trees(tree1.right, tree2.right)
+    return tree1
+
+
+def print_preorder(root: Optional[Node]) -> None:
+    """
+    Print pre-order traversal of the tree.
+
+    >>> root = Node(1)
+    >>> root.left = Node(2)
+    >>> root.right = Node(3)
+    >>> print_preorder(root)
+    1
+    2
+    3
+    >>> print_preorder(root.right)
+    3
+    """
+    if root:
+        print(root.value)
+        print_preorder(root.left)
+        print_preorder(root.right)
+
+
+if __name__ == "__main__":
+    tree1 = Node(1)
+    tree1.left = Node(2)
+    tree1.right = Node(3)
+    tree1.left.left = Node(4)
+
+    tree2 = Node(2)
+    tree2.left = Node(4)
+    tree2.right = Node(6)
+    tree2.left.right = Node(9)
+    tree2.right.right = Node(5)
+
+    print("Tree1 is: ")
+    print_preorder(tree1)
+    print("Tree2 is: ")
+    print_preorder(tree2)
+    merged_tree = merge_two_binary_trees(tree1, tree2)
+    print("Merged Tree is: ")
+    print_preorder(merged_tree)