Handle no-email accounts in account merge

Author: nickzerjeski

src/main/java/com/thealgorithms/graph/AccountMerge.java

@@ -41,6 +41,12 @@ public static List<List<String>> mergeAccounts(List<List<String>> accounts) {
             int root = dsu.find(entry.getValue());
             rootToEmails.computeIfAbsent(root, ignored -> new ArrayList<>()).add(entry.getKey());
         }
+        for (int i = 0; i < accounts.size(); i++) {
+            if (accounts.get(i).size() <= 1) {
+                int root = dsu.find(i);
+                rootToEmails.computeIfAbsent(root, ignored -> new ArrayList<>());
+            }
+        }
 
         List<List<String>> merged = new ArrayList<>();
         for (Map.Entry<Integer, List<String>> entry : rootToEmails.entrySet()) {
@@ -59,6 +65,9 @@ public static List<List<String>> mergeAccounts(List<List<String>> accounts) {
             if (cmp != 0) {
                 return cmp;
             }
+            if (a.size() == 1 || b.size() == 1) {
+                return Integer.compare(a.size(), b.size());
+            }
             return a.get(1).compareTo(b.get(1));
         });
         return merged;

src/test/java/com/thealgorithms/graph/AccountMergeTest.java

@@ -63,4 +63,20 @@ void testTransitiveMergeAndDuplicateEmails() {
 
         assertEquals(expected, merged);
     }
+
+    @Test
+    void testAccountsWithNoEmailsArePreserved() {
+        List<List<String>> accounts = List.of(
+            List.of("Alex"),
+            List.of("Alex", "alex1@mail.com"),
+            List.of("Bob"));
+
+        List<List<String>> merged = AccountMerge.mergeAccounts(accounts);
+        List<List<String>> expected = List.of(
+            List.of("Alex"),
+            List.of("Alex", "alex1@mail.com"),
+            List.of("Bob"));
+
+        assertEquals(expected, merged);
+    }
 }