N-ary Tree Postorder Traversal LeetCode solution in C++

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class Solution {
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 public:
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  void solve(Node *root, vector<int> &ans) {
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    if (root == NULL) return;
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    for (int i = 0; i < root->children.size(); i++) {
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      solve(root->children[i], ans);
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    }
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    ans.push_back(root->val);
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  }
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  vector<int> postorder(Node *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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  }