Flip Binary Tree To Match Preorder Traversal LeetCode solution in C++

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class Solution {
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 public:
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  bool helper(int &ind, TreeNode *root, vector<int> &voyage, vector<int> &ans) {
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    if (root == NULL || ind == voyage.size()) {
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      ind--;
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      return true;
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    }
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    // Not possible to create
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    if (root->val != voyage[ind]) {
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      ans.clear();
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      ans.push_back(-1);
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      return false;
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    }
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    // If voyage value not equal to its left child, then swap both childs and
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    // check
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    if (root->left && root->left->val != voyage[ind + 1]) {
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      TreeNode *temp = root->left;
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      root->left = root->right;
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      root->right = temp;
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      // Pusing root into ans
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      ans.push_back(root->val);
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    }
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    return helper(++ind, root->left, voyage, ans) && helper(++ind, root->right, voyage, ans);
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  }
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  vector<int> flipMatchVoyage(TreeNode *root, vector<int> &voyage) {
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    int ind = 0;
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    vector<int> ans;
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    helper(ind, root, voyage, ans);
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    return ans;
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  }
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};