Find the Closest Palindrome LeetCode solution in C++

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class Solution {
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 public:
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  string nearestPalindromic(string n) {
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    int l = n.size();
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    set<long> candidates;
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    // biggest, one more digit, 10...01
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    candidates.insert(long(pow(10, l)) + 1);
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    // smallest, one less digit, 9...9 or 0
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    candidates.insert(long(pow(10, l - 1)) - 1);
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    // the closest must be in middle digit +1, 0, -1, then flip left to right
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    long prefix = stol(n.substr(0, (l + 1) / 2));
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    for (long i = -1; i <= 1; ++i) {
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      string p = to_string(prefix + i);
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      string pp = p + string(p.rbegin() + (l & 1), p.rend());
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      candidates.insert(stol(pp));
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    }
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    long num = stol(n), minDiff = LONG_MAX, diff, minVal;
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    candidates.erase(num);
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    for (long val : candidates) {
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      diff = abs(val - num);
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      if (diff < minDiff) {
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        minDiff = diff;
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        minVal = val;
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      } else if (diff == minDiff) {
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        minVal = min(minVal, val);
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      }
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    }
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    return to_string(minVal);
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  }
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};