leet-code/3518/solution.cpp

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

using std::set;
using std::string;
using std::ranges::next_permutation;
using std::ranges::reverse;
using std::ranges::sort;

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

    string half = s.substr(0, s.size() / 2);

    sort(half);

    set<string> permutations;

    do {
      permutations.insert(half);
    } while (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 string();
    }

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

    string mirroredHalf = *it;
    reverse(mirroredHalf);

    return *it + mirroredHalf;
  }
};