Flatten Binary Tree to Linked List LeetCode solution in C++

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class Solution {
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 public:
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  TreeNode *prev = NULL;
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  void flatten(TreeNode *root) {
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    if (root == NULL) return;
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    flatten(root->right);
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    flatten(root->left);
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    root->right = prev;
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    root->left = NULL;
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    prev = root;
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  }
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};