564 - Find the Closest Palindrome

 JAVA

• class Solution {
      public String nearestPalindromic(String n) {
          long x = Long.parseLong(n);
          long ans = -1;
          for (long t : get(n)) {
              if (ans == -1 || Math.abs(t - x) < Math.abs(ans - x)
                  || (Math.abs(t - x) == Math.abs(ans - x) && t < ans)) {
                  ans = t;
              }
          }
          return Long.toString(ans);
      }
  
      private Set<Long> get(String n) {
          int l = n.length();
          Set<Long> res = new HashSet<>();
          res.add((long) Math.pow(10, l - 1) - 1);
          res.add((long) Math.pow(10, l) + 1);
          long left = Long.parseLong(n.substring(0, (l + 1) / 2));
          for (long i = left - 1; i <= left + 1; ++i) {
              StringBuilder sb = new StringBuilder();
              sb.append(i);
              sb.append(new StringBuilder(i + "").reverse().substring(l & 1));
              res.add(Long.parseLong(sb.toString()));
          }
          res.remove(Long.parseLong(n));
          return res;
      }
  }

C++

• class Solution {
  public:
      string nearestPalindromic(string n) {
          long x = stol(n);
          long ans = -1;
          for (long t : get(n))
              if (ans == -1 || abs(t - x) < abs(ans - x) || (abs(t - x) == abs(ans - x) && t < ans))
                  ans = t;
          return to_string(ans);
      }
  
      unordered_set<long> get(string& n) {
          int l = n.size();
          unordered_set<long> res;
          res.insert((long) pow(10, l - 1) - 1);
          res.insert((long) pow(10, l) + 1);
          long left = stol(n.substr(0, (l + 1) / 2));
          for (long i = left - 1; i <= left + 1; ++i) {
              string prefix = to_string(i);
              string t = prefix + string(prefix.rbegin() + (l & 1), prefix.rend());
              res.insert(stol(t));
          }
          res.erase(stol(n));
          return res;
      }
  };