/**
 * 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* track_back(TreeNode* root1, TreeNode* root2)
    {
        if(root1 == nullptr && root2 == nullptr) return nullptr;
        else if(root1 != nullptr && root2 == nullptr) 
        {
            TreeNode* cur = new TreeNode(root1->val);
            cur->left = track_back(root1->left,nullptr);
            cur->right = track_back(root1->right,nullptr);
            return cur;
        }
        else if(root1 == nullptr && root2 != nullptr)
        {
           TreeNode* cur = new TreeNode(root2->val);
            cur->left = track_back(nullptr,root2->left);
            cur->right = track_back(nullptr,root2->right);
            return cur ;
        }else if(root1 != nullptr && root2 != nullptr)
        {
            TreeNode* cur = new TreeNode(root1->val + root2->val);
            cur->left = track_back(root1->left,root2->left);
            cur->right = track_back(root1->right,root2->right);
            return cur;
        }
        return nullptr;
    }
    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
        TreeNode* root = track_back(root1,root2);
        return root;
    }
};