From 765b40ad712ac6b28c77aab16fa7692aaa2849fd Mon Sep 17 00:00:00 2001
From: Matej Focko <me@mfocko.xyz>
Date: Sun, 25 Aug 2024 14:05:50 +0200
Subject: [PATCH] =?UTF-8?q?cs:=20add=20=C2=AB564.=20Find=20the=20Closest?=
 =?UTF-8?q?=20Palindrome=C2=BB?=
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Signed-off-by: Matej Focko <me@mfocko.xyz>
---
 cs/find-the-closest-palindrome.cs | 56 +++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 cs/find-the-closest-palindrome.cs

diff --git a/cs/find-the-closest-palindrome.cs b/cs/find-the-closest-palindrome.cs
new file mode 100644
index 0000000..d6d3f6f
--- /dev/null
+++ b/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();
+    }
+}