Number of Good Leaf Nodes Pairs LeetCode solution in C++

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class Solution {
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 public:
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  int k, ans = 0;
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  int countPairs(TreeNode *root, int distance) {
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    k = distance;
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    cp(root, 0);
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    return ans;
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  }
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  vector<int> cp(TreeNode *root, int level) {
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    vector<int> v1;
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    if (root == NULL) {
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      return v1;
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    }
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    if (root->left == NULL && root->right == NULL) {
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      v1.push_back(level);
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      return v1;
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    }
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    v1 = cp(root->left, level + 1);
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    vector<int> v2 = cp(root->right, level + 1);
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    int a = v1.size();
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    int b = v2.size();
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    for (int i = 0; i < a; i++) {
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      for (int j = 0; j < b; j++) {
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        if (abs(v1[i] + v2[j] - (2 * level)) <= k) {
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          ans++;
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        }
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      }
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    }
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    int l = v2.size();
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    while (l--) {
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      v1.push_back(v2.back());
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      v2.pop_back();
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    }
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    return v1;
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  }
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};