Binary Tree Paths LeetCode solution in C++

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class Solution {
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 private:
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  void dfs(TreeNode *root, string s, vector<string> &ans) {
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    if (root == NULL) return;
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    s += (to_string(root->val));
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    if (root->left != NULL || root->right != NULL) s += "->";
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    if (root->left == NULL and root->right == NULL) {
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      ans.push_back(s);
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    }
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    dfs(root->left, s, ans);
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    dfs(root->right, s, ans);
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    s.pop_back();
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  }
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 public:
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  vector<string> binaryTreePaths(TreeNode *root) {
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    vector<string> ans;
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    string s;
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    dfs(root, s, ans);
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    return ans;
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  }
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};