Gray Code LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> grayCode(int n) {
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    vector<int> dp = {0, 1};
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    int cnt = 1;
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    for (int i = 2; i < n + 1; i++) {
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      int mod = pow(2, cnt);
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      int index = dp.size() - 1;
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      while (index >= 0) {
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        dp.push_back(dp[index] + mod);
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        index--;
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      }
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      cnt++;
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    }
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    return dp;
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  }
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};