Improved task 3646

Author: javadev

src/main/kotlin/g3601_3700/s3646_next_special_palindrome_number/Solution.kt

@@ -1,27 +1,29 @@
 package g3601_3700.s3646_next_special_palindrome_number
 
-// #Hard #Weekly_Contest_462 #2025_08_11_Time_1758_ms_(100.00%)_Space_70.14_MB_(100.00%)
+// #Hard #Weekly_Contest_462 #2025_08_12_Time_34_ms_(100.00%)_Space_53.80_MB_(66.67%)
 
 import java.util.Collections
 
 class Solution {
-    private val specials: MutableList<Long> = ArrayList<Long>()
+    companion object {
+        private val SPECIALS = mutableListOf<Long>()
+    }
 
     fun specialPalindrome(n: Long): Long {
-        if (specials.isEmpty()) {
-            init(specials)
+        if (SPECIALS.isEmpty()) {
+            init(SPECIALS)
         }
-        var pos = specials.binarySearch<Long>(n + 1)
+        var pos = SPECIALS.binarySearch(n + 1)
         if (pos < 0) {
             pos = -pos - 1
         }
-        return specials[pos]
+        return SPECIALS[pos]
     }
 
     private fun init(v: MutableList<Long>) {
-        val half: MutableList<Char> = ArrayList<Char>()
-        var mid: String?
-        for (mask in 1..<(1 shl 9)) {
+        val half = mutableListOf<Char>()
+        var mid: String
+        for (mask in 1 until (1 shl 9)) {
             var sum = 0
             var oddCnt = 0
             for (d in 1..9) {
@@ -40,19 +42,19 @@ class Solution {
             for (d in 1..9) {
                 if ((mask and (1 shl (d - 1))) != 0) {
                     if (d % 2 == 1) {
-                        mid = ('0'.code + d).toChar().toString()
+                        mid = ('0' + d).toString()
                     }
                     val h = d / 2
-                    for (i in 0..<h) {
-                        half.add(('0'.code + d).toChar())
+                    repeat(h) {
+                        half.add('0' + d)
                     }
                 }
             }
-            Collections.sort<Char>(half)
-            permute(half, 0, v, mid!!)
+            half.sort()
+            permute(half, 0, v, mid)
         }
-        v.sort<Long>()
-        val set: MutableSet<Long> = LinkedHashSet<Long>(v)
+        v.sort()
+        val set = LinkedHashSet(v)
         v.clear()
         v.addAll(set)
     }
@@ -63,17 +65,17 @@ class Solution {
             for (c in half) {
                 left.append(c)
             }
-            val right = StringBuilder(left).reverse().toString()
+            val right = left.reversed().toString()
             val s = left.toString() + mid + right
             if (s.isNotEmpty()) {
                 val x = s.toLong()
                 v.add(x)
             }
             return
         }
-        val swapped: MutableSet<Char?> = HashSet<Char?>()
-        for (i in start..<half.size) {
-            if (swapped.contains(half[i])) {
+        val swapped = mutableSetOf<Char>()
+        for (i in start until half.size) {
+            if (half[i] in swapped) {
                 continue
             }
             swapped.add(half[i])