1、題目：

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、解答：兩個結點的值相加而已，再遞迴的呼叫即可。

3、C++程式碼

class Solution {
public:
    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
        if(t1 && t2){
            TreeNode *root = new TreeNode(t1->val + t2->val);
            root->left = mergeTrees(t1->left,t2->left);
            root->right = mergeTrees(t1->right,t2->right);
            return root;
        }else{
            return t1?t1:t2;
        }
    }
};
 

python程式碼：

class Solution:
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if t1 and t2:
            root = TreeNode(t1.val+t2.val)
            root.left = self.mergeTrees(t1.left,t2.left)
            root.right = self.mergeTrees(t1.right,t2.right)
            return root
        else:
            return t1 or t2