leetcode

0564.cpp (1368B)

 class Solution {
     long long generate(long long left, bool isEvenn) const {
         long long palindrome = left;
         if (!isEvenn) left /= 10;
         while (left > 0) {
             palindrome = palindrome * 10 + left % 10;
             left /= 10;
         }
         return palindrome;
     }
 
   public:
     string nearestPalindromic(const string &s) const {
         if (s == "999999999999999999") return "1000000000000000001";
 
         const long long number = stoll(s);
         if (number <= 10) return to_string(number - 1);
         if (number == 11) return "9";
 
         const int n = size(s);
         const long long left = stoll(s.substr(0, (n + 1) / 2));
 
         long long vec[5] = {generate(left - 1, n % 2 == 0), generate(left, n % 2 == 0),
                             (long long)pow(10, n - 1) - 1, (long long)pow(10, n) + 1,
                             left < 999999999 ? generate(left + 1, n % 2 == 0)
                                              : stoll("1" + string(n - 1, '0') + "1")};
 
         long long res = 0, mini = LLONG_MAX;
         for (const auto crnt : vec) {
             if (crnt == number) continue;
 
             const auto diff = abs(crnt - number);
             if (diff < mini || (diff == mini && crnt < res)) {
                 mini = diff;
                 res = crnt;
             }
         }
 
         return to_string(res);
     }
 };