Max Sum of a Pair With Equal Sum of Digits LeetCode solution in C++

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class Solution {
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 public:
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  int maximumSum(vector<int> &nums) {
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    int ans = -1, sz = nums.size();
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    unordered_map<int, int> mp;
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    for (auto &i : nums) {
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      string s = to_string(i);
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      int sum = 0;
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      for (auto &ch : s) sum += (ch - '0');
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      if (mp.count(sum)) ans = max(ans, i + mp[sum]);
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      mp[sum] = max(i, mp[sum]);
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    }
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    return ans;
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  }
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};