[Google Question][LeetCode] Merge Two Binary Trees

Problem: You are given two binary trees root1 and root2.

Imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not. You need to merge the two trees into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of the new tree.

Return the merged tree.

Note: The merging process must start from the root nodes of both trees.

Example:

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]

Input: root1 = [1], root2 = [1,2]
Output: [2,2]

Approach: We can use Pre-Order traversal here. The approach is straight forward and can be understood by just looking at the implementation.

Implementation in C#:

        public void MergeBinaryTree(BinaryTree tree)

        {

            this.Root = this.MergeBinaryTree(this.Root, tree.Root);

        }

        private BinaryTreeNode MergeBinaryTree(BinaryTreeNode node1, BinaryTreeNode node2)

        {

            if (node1 == null)

            {

                return node2;

            }

            else if (node2 == null)

            {

                return node1;

            }

            node1.Value += node2.Value;

            node1.LeftNode = this.MergeBinaryTree(node1.LeftNode, node2.LeftNode);

            node1.RightNode = this.MergeBinaryTree(node1.RightNode, node2.RightNode);

            return node1;

        } 

Complexity: O(n)