题目：
题解：
cpp
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
if(root1 && root2){
root1->val += root2->val;
// 直接在 root1 上合并，所以这里递归的节点都是root1
root1->left = mergeTrees(root1->left,root2->left);
root1->right = mergeTrees(root1->right,root2->right);
}
return root1 ? root1 : root2;
}
};