Merge Two Binary Trees LeetCode solution in C++

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/**
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 * Definition for a binary tree node.
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 * struct TreeNode {
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 * int val;
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 * TreeNode *left;
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 * TreeNode *right;
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 * TreeNode() : val(0), left(nullptr), right(nullptr) {}
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 * TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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 * TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
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 * right(right) {}
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 * };
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 */
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class Solution {
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 public:
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  TreeNode *mergeTrees(TreeNode *root1, TreeNode *root2) {
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    if (root1 == NULL) return root2;
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    if (root2 == NULL) return root1;
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    root1->val += root2->val;
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    root1->left = mergeTrees(root1->left, root2->left);
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    root1->right = mergeTrees(root1->right, root2->right);
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    return root1;
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  }
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};