cs: add «564. Find the Closest Palindrome»

Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/find-the-closest-palindrome.cs

@@ -0,0 +1,56 @@
+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();
+    }
+}