diff --git a/src/main/kotlin/g1001_1100/s1044_longest_duplicate_substring/Solution.kt b/src/main/kotlin/g1001_1100/s1044_longest_duplicate_substring/Solution.kt
index ed26160a2..52b05204f 100644
--- a/src/main/kotlin/g1001_1100/s1044_longest_duplicate_substring/Solution.kt
+++ b/src/main/kotlin/g1001_1100/s1044_longest_duplicate_substring/Solution.kt
@@ -6,7 +6,7 @@ package g1001_1100.s1044_longest_duplicate_substring
 class Solution {
     private lateinit var hsh: LongArray
     private lateinit var pw: LongArray
-    private val cnt: Array<MutableList<Int>?> = arrayOfNulls(26)
+    private val cnt: Array<MutableList<Int>> = Array(26) { ArrayList() }
 
     fun longestDupSubstring(s: String): String {
         val n = s.length
@@ -20,17 +20,17 @@ class Solution {
         for (j in 1..n) {
             hsh[j] = (hsh[j - 1] * base + s[j - 1].code.toLong()) % MOD
             pw[j] = pw[j - 1] * base % MOD
-            cnt[s[j - 1].code - 'a'.code]!!.add(j - 1)
+            cnt[s[j - 1].code - 'a'.code].add(j - 1)
         }
         var ans = ""
         for (i in 0..25) {
-            if (cnt[i]!!.isEmpty()) {
+            if (cnt[i].isEmpty()) {
                 continue
             }
-            val idx: MutableList<Int>? = cnt[i]
-            var set: MutableSet<Long?>
+            val idx: MutableList<Int> = cnt[i]
+            var set: MutableSet<Long>
             var lo = 1
-            var hi = n - idx!![0]
+            var hi = n - idx[0]
             while (lo <= hi) {
                 val len = (lo + hi) / 2
                 set = HashSet()
diff --git a/src/main/kotlin/g2501_2600/s2551_put_marbles_in_bags/Solution.kt b/src/main/kotlin/g2501_2600/s2551_put_marbles_in_bags/Solution.kt
index 173ed8300..fae1a48d6 100644
--- a/src/main/kotlin/g2501_2600/s2551_put_marbles_in_bags/Solution.kt
+++ b/src/main/kotlin/g2501_2600/s2551_put_marbles_in_bags/Solution.kt
@@ -7,27 +7,27 @@ import java.util.PriorityQueue
 
 class Solution {
     fun putMarbles(weights: IntArray, k: Int): Long {
-        // Map<Pair<Integer, Integer>, long[]> memo = new HashMap<>();
-        // long[] res = dfs(weights, 0, k, memo);
-        // return res[1] - res[0];
-        if (k == 1 || k == weights.size) return 0
+        if (k == 1 || k == weights.size) {
+            return 0
+        }
         val min = PriorityQueue<Long>()
-        val max = PriorityQueue { a: Long?, b: Long? ->
-            java.lang.Long.compare(
-                b!!,
-                a!!,
-            )
+        val max = PriorityQueue { a: Long, b: Long ->
+            b.compareTo(a)
         }
         for (i in 0 until weights.size - 1) {
             val sum = weights[i].toLong() + weights[i + 1]
             min.offer(sum)
             max.offer(sum)
-            if (min.size == k) min.poll()
-            if (max.size == k) max.poll()
+            if (min.size == k) {
+                min.poll()
+            }
+            if (max.size == k) {
+                max.poll()
+            }
         }
         var res: Long = 0
         while (max.isNotEmpty()) {
-            res += min.poll() - max.poll()!!
+            res += min.poll() - max.poll()
         }
         return res
     }
diff --git a/src/main/kotlin/g3301_3400/s3367_maximize_sum_of_weights_after_edge_removals/Solution.kt b/src/main/kotlin/g3301_3400/s3367_maximize_sum_of_weights_after_edge_removals/Solution.kt
index 8beb4e525..62f7c6756 100644
--- a/src/main/kotlin/g3301_3400/s3367_maximize_sum_of_weights_after_edge_removals/Solution.kt
+++ b/src/main/kotlin/g3301_3400/s3367_maximize_sum_of_weights_after_edge_removals/Solution.kt
@@ -26,7 +26,7 @@ class Solution {
 
     private fun dfs(v: Int, parent: Int): LongArray {
         var sum: Long = 0
-        val pq = PriorityQueue<Long?>()
+        val pq = PriorityQueue<Long>()
         for (e in adj[v]) {
             val w = if (e[0] == v) e[1] else e[0]
             if (w == parent) {