leet-code/3518/solution.cpp

#include <algorithm>
#include <limits>
#include <set>
#include <stdexcept>
#include <string>

class Solution {
public:
  std::string smallestPalindrome(std::string s, int k) {

    std::string half = s.substr(0, s.size() / 2);
    char middle = (s.size() % 2 == 1) ? s[s.size() / 2] : '\0';

    std::ranges::sort(half);

    std::set<std::string> permutations;

    do {
      permutations.insert(half);
    } while (std::ranges::next_permutation(half).found);

    if (permutations.size() > std::numeric_limits<int>::max()) {
      throw std::overflow_error("Number of permutations exceeds int limit");
    }
    int size = static_cast<int>(permutations.size());

    if (k > size) {
      return std::string();
    }

    auto it = permutations.begin();
    std::advance(it, k - 1);

    std::string mirroredHalf = *it;
    std::ranges::reverse(mirroredHalf);

    if (middle != '\0') {
      return *it + std::string(1, middle) + mirroredHalf;
    }

    return *it + mirroredHalf;
  }
};