diff --git a/solutions/add-two-numbers/Solution.java b/solutions/add-two-numbers/Solution.java
index 807856b..086d6da 100644
--- a/solutions/add-two-numbers/Solution.java
+++ b/solutions/add-two-numbers/Solution.java
@@ -3,16 +3,17 @@
 // Memory Usage: 39.3 MB
 // Submitted: January 15th, 2021
 
-/**
- * Definition for singly-linked list.
- * public class ListNode {
- *     int val;
- *     ListNode next;
- *     ListNode() {}
- *     ListNode(int val) { this.val = val; }
- *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
- * }
- */
+class ListNode {
+    int val;
+    ListNode next;
+    ListNode() {}
+    ListNode(int val) { this.val = val; }
+    ListNode(int val, ListNode next) {
+        this.val = val;
+        this.next = next;
+    }
+}
+
 class Solution {
     public ListNode addTwoNumbers(ListNode a, ListNode b) {
         ListNode dummy = new ListNode(0);
diff --git a/solutions/first-unique-character-in-a-string/Solution_HashMap.java b/solutions/first-unique-character-in-a-string/Solution_HashMap.java
index 4f5b035..168deba 100644
--- a/solutions/first-unique-character-in-a-string/Solution_HashMap.java
+++ b/solutions/first-unique-character-in-a-string/Solution_HashMap.java
@@ -3,6 +3,9 @@
 // Memory Usage: 39.5 MB
 // Submitted: January 14th, 2021
 
+import java.util.Map;
+import java.util.HashMap;
+
 class Solution {
     public int firstUniqChar(String s) {
         Map<Character, Integer> count = new HashMap<Character, Integer>();
diff --git a/solutions/longest-string-chain/Solution.java b/solutions/longest-string-chain/Solution.java
index 367296d..d7b5ef2 100644
--- a/solutions/longest-string-chain/Solution.java
+++ b/solutions/longest-string-chain/Solution.java
@@ -3,6 +3,10 @@
 // Memory Usage: 39.1 MB
 // Submitted: January 14th, 2021
 
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+
 class Solution {
     public int longestStrChain(String[] words) {
         Map<String, Integer> chain = new HashMap<String, Integer>();
diff --git a/solutions/longest-substring-without-repeating-characters/Solution.java b/solutions/longest-substring-without-repeating-characters/Solution.java
index 52cd5f4..3061b96 100644
--- a/solutions/longest-substring-without-repeating-characters/Solution.java
+++ b/solutions/longest-substring-without-repeating-characters/Solution.java
@@ -3,6 +3,9 @@
 // Memory Usage: 39.1 MB
 // Submitted: January 13th, 2021
 
+import java.util.HashMap;
+import java.util.Map;
+
 class Solution {
     public int lengthOfLongestSubstring(String s) {
         int max = 0;
diff --git a/solutions/making-file-names-unique/Solution.java b/solutions/making-file-names-unique/Solution.java
index 5ed4b60..3e6b888 100644
--- a/solutions/making-file-names-unique/Solution.java
+++ b/solutions/making-file-names-unique/Solution.java
@@ -3,8 +3,7 @@
 // Memory Usage: 54.7 MB
 // Submitted: January 14th, 2021
 
-import java.util.regex.Matcher;
-import java.util.regex.Pattern;
+import java.util.HashMap;
 
 class Solution {
     HashMap<String, Integer> count = new HashMap<String, Integer>();
diff --git a/solutions/making-file-names-unique/Solution_regex.java b/solutions/making-file-names-unique/Solution_regex.java
index 28f99cb..97ecdba 100644
--- a/solutions/making-file-names-unique/Solution_regex.java
+++ b/solutions/making-file-names-unique/Solution_regex.java
@@ -6,6 +6,12 @@
 // that worked, but it also used RegEx, did operations it didn't need to, and overall was much too complex and aggressive to work well.
 // The correct solution was much simpler and more concise. It did technically work, though!
 
+import java.util.Map;
+import java.util.Set;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.ArrayList;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
diff --git a/solutions/roman-to-integer/Solution.java b/solutions/roman-to-integer/Solution.java
index f64061b..db66352 100644
--- a/solutions/roman-to-integer/Solution.java
+++ b/solutions/roman-to-integer/Solution.java
@@ -3,6 +3,9 @@
 // Memory Usage: 39.3 MB
 // Submitted: January 15th, 2021
 
+import java.util.HashMap;
+import java.util.Map;
+
 class Solution {
     Map<Character, Integer> convert = new HashMap<Character, Integer>();
     
diff --git a/solutions/sort-array-by-increasing-frequency/Solution.java b/solutions/sort-array-by-increasing-frequency/Solution.java
index 9fda972..92a09d8 100644
--- a/solutions/sort-array-by-increasing-frequency/Solution.java
+++ b/solutions/sort-array-by-increasing-frequency/Solution.java
@@ -3,6 +3,10 @@
 // Memory Usage: 39.1 MB
 // Submitted: January 15th, 2021
 
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+
 class Solution {
     public int[] frequencySort(int[] nums) {
         Map<Integer, Integer> count = new HashMap<Integer, Integer>();
diff --git a/solutions/sort-characters-by-frequency/Solution.java b/solutions/sort-characters-by-frequency/Solution.java
index 422e6fd..fe65c3f 100644
--- a/solutions/sort-characters-by-frequency/Solution.java
+++ b/solutions/sort-characters-by-frequency/Solution.java
@@ -3,6 +3,10 @@
 // Memory Usage: 40 MB
 // Submitted: January 14th, 2021
 
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+
 class Solution {
     public String frequencySort(String s) {
         // Get each character's frequency
diff --git a/solutions/two-sum/Solution.java b/solutions/two-sum/Solution.java
index a4705e1..03cb1ce 100644
--- a/solutions/two-sum/Solution.java
+++ b/solutions/two-sum/Solution.java
@@ -3,6 +3,9 @@
 // Memory Usage: 39.7 MB
 // Submitted: January 12th, 2021
 
+import java.util.HashMap;
+import java.util.Map;
+
 class Solution {
     public int[] twoSum(int[] nums, int target) {
         Map<Integer, Integer> map = new HashMap<>();