diff --git a/src/main/kotlin/g0901_1000/s0989_add_to_array_form_of_integer/Solution.kt b/src/main/kotlin/g0901_1000/s0989_add_to_array_form_of_integer/Solution.kt
index d5ab2e95d..7ce338a92 100644
--- a/src/main/kotlin/g0901_1000/s0989_add_to_array_form_of_integer/Solution.kt
+++ b/src/main/kotlin/g0901_1000/s0989_add_to_array_form_of_integer/Solution.kt
@@ -5,9 +5,9 @@ package g0901_1000.s0989_add_to_array_form_of_integer
 
 @Suppress("NAME_SHADOWING")
 class Solution {
-    fun addToArrayForm(num: IntArray, k: Int): List<Int?> {
+    fun addToArrayForm(num: IntArray, k: Int): List<Int> {
         var k = k
-        val result = ArrayList<Int?>()
+        val result = ArrayList<Int>()
         var carry = 0
         for (i in num.indices.reversed()) {
             val temp = num[i] + k % 10 + carry
diff --git a/src/main/kotlin/g1001_1100/s1048_longest_string_chain/Solution.kt b/src/main/kotlin/g1001_1100/s1048_longest_string_chain/Solution.kt
index 63650c1a3..1a085c5d0 100644
--- a/src/main/kotlin/g1001_1100/s1048_longest_string_chain/Solution.kt
+++ b/src/main/kotlin/g1001_1100/s1048_longest_string_chain/Solution.kt
@@ -5,7 +5,7 @@ package g1001_1100.s1048_longest_string_chain
 
 class Solution {
     fun longestStrChain(words: Array<String>): Int {
-        val lenStr = arrayOfNulls<MutableList<String>?>(20)
+        val lenStr = arrayOfNulls<MutableList<String>>(20)
         for (word in words) {
             val len = word.length
             if (lenStr[len] == null) {
diff --git a/src/main/kotlin/g1601_1700/s1639_number_of_ways_to_form_a_target_string_given_a_dictionary/Solution.kt b/src/main/kotlin/g1601_1700/s1639_number_of_ways_to_form_a_target_string_given_a_dictionary/Solution.kt
index 162d456bd..e92e2eb53 100644
--- a/src/main/kotlin/g1601_1700/s1639_number_of_ways_to_form_a_target_string_given_a_dictionary/Solution.kt
+++ b/src/main/kotlin/g1601_1700/s1639_number_of_ways_to_form_a_target_string_given_a_dictionary/Solution.kt
@@ -6,7 +6,7 @@ package g1601_1700.s1639_number_of_ways_to_form_a_target_string_given_a_dictiona
 class Solution {
     fun numWays(words: Array<String>, target: String): Int {
         val counts = precompute(words)
-        val memo = Array(target.length) { arrayOfNulls<Int?>(words[0].length) }
+        val memo = Array(target.length) { arrayOfNulls<Int>(words[0].length) }
         return solve(memo, counts, words, target, 0, 0)
     }
 
diff --git a/src/main/kotlin/g2001_2100/s2019_the_score_of_students_solving_math_expression/Solution.kt b/src/main/kotlin/g2001_2100/s2019_the_score_of_students_solving_math_expression/Solution.kt
index be0744309..74fa458ae 100644
--- a/src/main/kotlin/g2001_2100/s2019_the_score_of_students_solving_math_expression/Solution.kt
+++ b/src/main/kotlin/g2001_2100/s2019_the_score_of_students_solving_math_expression/Solution.kt
@@ -12,7 +12,7 @@ class Solution {
         val st = ArrayDeque<Int>()
         val n = s.length
         var i = 0
-        dp = Array(n) { arrayOfNulls<HashSet<Int>?>(n) }
+        dp = Array(n) { arrayOfNulls<HashSet<Int>>(n) }
         while (i < n) {
             if (s[i].code - '0'.code >= 0 && s[i].code - '9'.code <= 0) {
                 st.push(s[i].code - '0'.code)
diff --git a/src/main/kotlin/g2401_2500/s2452_words_within_two_edits_of_dictionary/Solution.kt b/src/main/kotlin/g2401_2500/s2452_words_within_two_edits_of_dictionary/Solution.kt
index 04ff57650..7632aeb71 100644
--- a/src/main/kotlin/g2401_2500/s2452_words_within_two_edits_of_dictionary/Solution.kt
+++ b/src/main/kotlin/g2401_2500/s2452_words_within_two_edits_of_dictionary/Solution.kt
@@ -6,7 +6,7 @@ class Solution {
     private var root: Node? = null
 
     internal class Node {
-        var childs = HashMap<Char, Node?>()
+        var childs = HashMap<Char, Node>()
     }
 
     private fun insert(s: String) {
diff --git a/src/main/kotlin/g3201_3300/s3245_alternating_groups_iii/Solution.kt b/src/main/kotlin/g3201_3300/s3245_alternating_groups_iii/Solution.kt
index 3544457b3..87e95c933 100644
--- a/src/main/kotlin/g3201_3300/s3245_alternating_groups_iii/Solution.kt
+++ b/src/main/kotlin/g3201_3300/s3245_alternating_groups_iii/Solution.kt
@@ -5,7 +5,7 @@ package g3201_3300.s3245_alternating_groups_iii
 import java.util.BitSet
 
 class Solution {
-    fun numberOfAlternatingGroups(colors: IntArray, queries: Array<IntArray>): MutableList<Int?> {
+    fun numberOfAlternatingGroups(colors: IntArray, queries: Array<IntArray>): List<Int> {
         val n = colors.size
         val set = BitSet()
         val bit = BIT(n)
@@ -14,7 +14,7 @@ class Solution {
                 add(set, bit, n, i)
             }
         }
-        val ans: MutableList<Int?> = ArrayList<Int?>()
+        val ans: MutableList<Int> = ArrayList<Int>()
         for (q in queries) {
             if (q[0] == 1) {
                 if (set.isEmpty) {
diff --git a/src/main/kotlin/g3201_3300/s3283_maximum_number_of_moves_to_kill_all_pawns/Solution.kt b/src/main/kotlin/g3201_3300/s3283_maximum_number_of_moves_to_kill_all_pawns/Solution.kt
index e988b4a3e..3acfc534d 100644
--- a/src/main/kotlin/g3201_3300/s3283_maximum_number_of_moves_to_kill_all_pawns/Solution.kt
+++ b/src/main/kotlin/g3201_3300/s3283_maximum_number_of_moves_to_kill_all_pawns/Solution.kt
@@ -61,10 +61,10 @@ class Solution {
         if (x1 == x2 && y1 == y2) {
             return 0
         }
-        val visited = Array<BooleanArray?>(50) { BooleanArray(50) }
+        val visited = Array<BooleanArray>(50) { BooleanArray(50) }
         val queue: Queue<IntArray> = LinkedList<IntArray>()
         queue.offer(intArrayOf(x1, y1, 0))
-        visited[x1]!![y1] = true
+        visited[x1][y1] = true
         while (queue.isNotEmpty()) {
             val current = queue.poll()
             val x = current[0]
@@ -76,9 +76,9 @@ class Solution {
                 if (nx == x2 && ny == y2) {
                     return moves + 1
                 }
-                if (nx >= 0 && nx < 50 && ny >= 0 && ny < 50 && !visited[nx]!![ny]) {
+                if (nx >= 0 && nx < 50 && ny >= 0 && ny < 50 && !visited[nx][ny]) {
                     queue.offer(intArrayOf(nx, ny, moves + 1))
-                    visited[nx]!![ny] = true
+                    visited[nx][ny] = true
                 }
             }
         }
diff --git a/src/main/kotlin/g3401_3500/s3486_longest_special_path_ii/Solution.kt b/src/main/kotlin/g3401_3500/s3486_longest_special_path_ii/Solution.kt
index 5b6f4142f..f62a0ed3a 100644
--- a/src/main/kotlin/g3401_3500/s3486_longest_special_path_ii/Solution.kt
+++ b/src/main/kotlin/g3401_3500/s3486_longest_special_path_ii/Solution.kt
@@ -53,7 +53,7 @@ class Solution {
             if (last.containsKey(nums[nextNode])) {
                 nextLeft.add(last[nums[nextNode]]!! + 1)
             }
-            nextLeft.sortWith(Comparator.naturalOrder<Int?>())
+            nextLeft.sortWith(Comparator.naturalOrder<Int>())
             while (nextLeft.size > 2) {
                 nextLeft.removeAt(0)
             }
diff --git a/src/main/kotlin/g3401_3500/s3490_count_beautiful_numbers/Solution.kt b/src/main/kotlin/g3401_3500/s3490_count_beautiful_numbers/Solution.kt
index 24e820d04..3bde7d146 100644
--- a/src/main/kotlin/g3401_3500/s3490_count_beautiful_numbers/Solution.kt
+++ b/src/main/kotlin/g3401_3500/s3490_count_beautiful_numbers/Solution.kt
@@ -9,7 +9,7 @@ class Solution {
 
     private fun countBeautiful(x: Int): Int {
         val digits = getCharArray(x)
-        val dp = HashMap<String?, Int?>()
+        val dp = HashMap<String, Int>()
         return solve(0, 1, 0, 1, digits, dp)
     }
 
@@ -24,7 +24,7 @@ class Solution {
         sum: Int,
         prod: Int,
         digits: CharArray,
-        dp: HashMap<String?, Int?>,
+        dp: HashMap<String, Int>,
     ): Int {
         if (i == digits.size) {
             return if (sum > 0 && prod % sum == 0) {