Check Completeness of a Binary Tree LeetCode solution in C++

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class Solution {
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 public:
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  bool isComplete = true;
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  // Returns min, max height of node
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  pair<int, int> helper(TreeNode *root) {
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    // NULL is of height zero
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    if (root == NULL) {
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      return {0, 0};
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    }
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    // Moving to its children
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    pair<int, int> lst = helper(root->left);
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    pair<int, int> rst = helper(root->right);
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    // Height of rst is greater than lst or
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    // There are levels which are not filled other than the last level (diff
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    // between levels > 1)
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    if (rst.second > lst.first || lst.second - rst.first > 1) {
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      isComplete = false;
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    }
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    return {min(lst.first, rst.first) + 1, max(lst.second, rst.second) + 1};
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  }
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  bool isCompleteTree(TreeNode *root) {
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    helper(root);
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    return isComplete;
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  }
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};