Binary Tree Preorder Traversal LeetCode solution in C++

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class Solution {
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  void solve(TreeNode *root, vector<int> &ans) {
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    if (root == NULL) return;
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    ans.push_back(root->val);
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    solve(root->left, ans);
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    solve(root->right, ans);
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  }
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 public:
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  vector<int> preorderTraversal(TreeNode *root) {
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    vector<int> ans;
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    solve(root, ans);
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    return ans;
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  }
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};