fixes #834 (#1021)

Author: martin-henz

xml/chapter3/section3/subsection2.xml

@@ -951,6 +951,98 @@ print_queue(q); // prints: ["b", null]
     All operations should be accomplished in
     <LATEXINLINE>$\Theta(1)$</LATEXINLINE> steps.
     <LABEL NAME="ex:deque"/>
+    <SOLUTION>
+      <SNIPPET>
+	<JAVASCRIPT>
+// solution provided by GitHub user clean99
+function make_deque() {
+    return pair(null, null);
+}
+
+function front_ptr(deque) {
+    return head(deque);
+}
+
+function rear_ptr(deque) {
+    return tail(deque);
+}
+
+function set_front_ptr(deque, item) {
+    set_head(deque, item);
+}
+
+function set_rear_ptr(deque, item) {
+    set_tail(deque, item);
+}
+
+function is_empty_deque(deque) {
+    return is_null(front_ptr(deque))
+           ? true
+  	   : is_null(rear_ptr(deque))
+  	   ? true
+  	   : false;
+}
+
+function is_one_item_deque(deque) {
+    return front_ptr(deque) === rear_ptr(deque);
+}
+
+function front_insert_deque(deque, item) {
+    // use another pair to store a forward pointer
+    const new_pair = pair(pair(item, null), null);
+    if (is_empty_deque(deque)) {
+        set_front_ptr(deque, new_pair);
+        set_rear_ptr(deque, new_pair);
+    } else {
+        set_tail(new_pair, front_ptr(deque));
+        // set forward pointer to new_pair
+        set_tail(head(front_ptr(deque)), new_pair);
+        set_front_ptr(deque, new_pair);
+    }
+}
+
+function front_delete_deque(deque) {
+    if (is_empty_deque(deque)) {
+        error(deque, "front_delete_deque called with an empty deque");
+    } else if(is_one_item_deque(deque)) {
+        set_front_ptr(deque, null);
+        set_rear_ptr(deque, null);
+        return deque;
+    } else {
+        set_front_ptr(deque, tail(front_ptr(deque)));
+        return deque;
+    }
+}
+
+function rear_insert_deque(deque, item) {
+    if (is_empty_deque(deque)) {
+        const new_pair = pair(pair(item, null), null);
+        set_front_ptr(deque, new_pair);
+        set_rear_ptr(deque, new_pair);
+    } else {
+        // set new_pair forward pointer to last item
+        const new_pair = pair(pair(item, rear_ptr(deque)), null);
+        set_tail(rear_ptr(deque), new_pair);
+        set_rear_ptr(deque, new_pair);
+    }
+}
+
+function rear_delete_deque(deque) {
+    if (is_empty_deque(deque)) {
+        error(deque, "rear_delete_deque called with an empty deque");
+    } else if(is_one_item_deque(deque)) {
+        set_front_ptr(deque, null);
+        set_rear_ptr(deque, null);
+        return deque;
+    } else {
+        // update rear_ptr to last item's forward pointer
+        set_rear_ptr(deque, tail(head(rear_ptr(deque))));
+        return deque;
+    }
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
   <INDEX>queue<CLOSE/></INDEX>
 </SUBSECTION>