fixes #804 (#1011)

* fixes #804

* credit

Author: martin-henz

xml/chapter3/section3/subsection2.xml

@@ -738,6 +738,34 @@ delete_queue(q1);
     </SPLITINLINE>
     that takes a queue as input and prints the sequence of items in the queue.
     <LABEL NAME="ex:3_21"/>
+    <SOLUTION>
+      <SNIPPET>
+	<REQUIRES>make_queue</REQUIRES>
+	<REQUIRES>modify_pointers</REQUIRES>
+	<REQUIRES>insert_queue</REQUIRES>
+	<REQUIRES>is_empty_queue</REQUIRES>
+	<REQUIRES>delete_queue</REQUIRES>
+	<EXAMPLE>ex_3_21_solution_example</EXAMPLE>
+	<JAVASCRIPT>
+function print_queue(q) {
+    return display(head(q));
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+	<NAME>ex_3_21_solution_example</NAME>
+	<JAVASCRIPT>
+const q1 = make_queue();
+print_queue(q1); // prints: null
+insert_queue(q1, "a");
+print_queue(q1); // prints: ["a", null]
+insert_queue(q1, "b");
+print_queue(q1); // prints: ["a", ["b", null]]
+delete_queue(q1);
+print_queue(q1); // prints: ["b", null]
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>
@@ -786,6 +814,76 @@ function make_queue() {
     and provide implementations of the queue operations using this
     representation.
     <LABEL NAME="ex:3_22"/>
+    <SOLUTION>
+      <SNIPPET>
+	<EXAMPLE>ex_3_22_example</EXAMPLE>
+	<JAVASCRIPT>
+// provided by GitHub user devinryu	  
+	  
+function make_queue() {
+    let front_ptr = null;
+    let rear_ptr = null;
+    function is_empty_queue() {
+        return is_null(front_ptr);
+    }
+    function insert_queue(item) {
+        let new_pair = pair(item, null);
+        if (is_empty_queue()) {
+            front_ptr = new_pair;
+            rear_ptr = new_pair;
+        } else {
+            set_tail(rear_ptr, new_pair);
+            rear_ptr = new_pair;
+        }
+    }
+    function delete_queue() {
+        if (is_empty_queue()) {
+            error("FRONT called with an empty queue");
+        } else {
+            front_ptr = tail(front_ptr);
+        }
+    }
+    function print_queue() {
+        display(front_ptr);
+    }
+    function dispatch(m) {
+        return m === "insert_queue"
+        ? insert_queue
+        : m === "delete_queue"
+        ? delete_queue
+        : m === "is_empty_queue"
+        ? is_empty_queue
+        : m === "print_queue"
+        ? print_queue
+        : error(m, "Unknow operation -- DISPATCH");
+    }
+    return dispatch;
+}
+function insert_queue(queue, item) {
+    return queue("insert_queue")(item);
+}
+function delete_queue(queue) {
+    return queue("delete_queue")();
+}
+function print_queue(queue) {
+    return queue("print_queue")();
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+	<NAME>ex_3_22_example</NAME>
+	<JAVASCRIPT>
+const q = make_queue();
+print_queue(q); // prints: null
+insert_queue(q, "a");
+print_queue(q); // prints: ["a", null]
+insert_queue(q, "b");
+print_queue(q); // prints: ["a", ["b", null]]
+delete_queue(q);
+print_queue(q); // prints: ["b", null]
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>