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# 617 Merge Two Binary Trees

## 617. Merge Two Binary Trees

## 1. Question

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.

## 2. Implementation

**(1) Recursion**

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

        if (t2 == null) {
            return t1;
        }

        t1.val += t2.val;
        t1.left = mergeTrees(t1.left, t2.left);
        t1.right = mergeTrees(t1.right, t2.right);
        return t1;
    }
}
```

## 3. Time & Space Complexity

Recursion：时间复杂度是O(N), N 为输入两个树的节点综合，空间复杂度为O(N), 当两棵树都是skewed的时候，平均是O(logN)