Implement exchangePairs

src/aud/exam/prep/list/RecursiveDoubleListItemProcessor.java

@@ -94,8 +94,26 @@ public class RecursiveDoubleListItemProcessor<T> extends DoubleListItemProcessor
 
     @Override
     public void exchangePairs(DoubleListItem<T> list) {
-        // TODO: Your training
+        if (list == null || list.next == null || list.next.next == null) {
-        throw new RuntimeException("unimplemented");
+            return;
+        }
+
+        var next = list.next;
+        var nextNext = next.next;
+        var rest = nextNext.next;
+
+        list.next = nextNext;
+        nextNext.prev = list;
+
+        nextNext.next = next;
+        next.prev = nextNext;
+
+        next.next = rest;
+        if (rest != null) {
+            rest.prev = next;
+        }
+
+        exchangePairs(list.next.next);
     }
 
     @Override