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.

Example1

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

Example2

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

SOLVING

We’ll use Depth-first Search method

Steps

• If root1 and root2 are null return root1
• If root2 is null return root1
• If root1 is null return root2
• Else:
  • root1 value += root2 value
  • root1 left is equal to Recursive as params root1 left and root2 left
  • root1 right is equal to Recursive as params root1 right and root2 right
• Return root1

Code

class Solution {
public:
  TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
    if (!root1 && !root2) return root1;
    if (!root1) return root2;
    if (!root2) return root1;

    root1->val += root2->val;
    root1->left = mergeTrees(root1->left, root2->left);
    root1->right = mergeTrees(root1->right, root2->right);
    return root1;
  }
};

• DEPTHFIRSTSEARCH
• PROBLEM