LeetCode/cs/find-the-closest-palindrome.cs

public class Solution {
    private static long MakePalindrome(long num) {
        var s = num.ToString();
        var length = s.Length;

        var digits = s.ToArray();
        for (int l = (length - 1) / 2, r = length / 2; l >= 0; --l, ++r) {
            digits[r] = digits[l];
        }

        return long.Parse(new string(digits));
    }

    private static long FindPalindrome(
        long left,
        long right,
        Func<long, long, long, long, (bool, long, long)> update
    ) {
        var ans = long.MinValue;

        bool foundAnswer;
        while (left <= right) {
            var mid = left + (right - left) / 2;
            var found = MakePalindrome(mid);

            (foundAnswer, left, right) = update(left, right, found, mid);
            if (foundAnswer) {
                ans = found;
            }
        }

        return ans;
    }

    public string NearestPalindromic(string n) {
        var number = long.Parse(n);

        var prev = FindPalindrome(0, number, (long left, long right, long found, long mid) => {
            if (found < number) {
                return (true, mid + 1, right);
            }
            return (false, left, mid - 1);
        });
        var next = FindPalindrome(number, (long)1e18, (long left, long right, long found, long mid) => {
            if (found > number) {
                return (true, left, mid - 1);
            }
            return (false, mid + 1, right);
        });

        if (Math.Abs(prev - number) <= Math.Abs(next - number)) {
            return prev.ToString();
        }
        return next.ToString();
    }
}