# 617. Merge Two Binary Trees (Easy)

https://leetcode.com/problems/merge-two-binary-trees/

Given two binary trees and 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 them 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 new tree.

Example 1:

```Input:
Tree 1                     Tree 2
1                         2
/ \                       / \
3   2                     1   3
/                           \   \
5                             4   7
Output:
Merged tree:
3
/ \
4   5
/ \   \
5   4   7
```

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

## Solutions

``````class Solution {
public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
// both null
if (t1 == null && t2 == null) {
return null;
}

// t1 null, t2 not null, return immediately since no need to go deeper
if (t1 == null && t2 != null) {
return t2;
}

// t1 not null, t2 null, return immediately since no need to go deeper
if (t1 != null && t2 == null) {
return t1;
}

// both not null
TreeNode node = new TreeNode(t1.val + t2.val);

// merge left branch
node.left = mergeTrees(t1.left, t2.left);

// merge right branch
node.right = mergeTrees(t1.right, t2.right);

return node;
}
}
``````