All tests pass

src/LongStack.java

@@ -1,51 +1,111 @@
+import java.util.Iterator;
+import java.util.LinkedList;
 
 public class LongStack {
 
+   private final LinkedList<Long> lifo;
+
    public static void main (String[] argum) {
-      // TODO!!! Your tests here!
+      long i = interpret("1 -2 45 -45");
+
    }
 
    LongStack() {
-      // TODO!!! Your constructor here!
+      this.lifo = new LinkedList<>();
+   }
+
+   LongStack(LinkedList<Long> lifo) {
+      this.lifo = lifo;
    }
 
    @Override
    public Object clone() throws CloneNotSupportedException {
-      return this; // TODO!!! Your code here!
+      return new LongStack((LinkedList<Long>) this.lifo.clone());
    }
 
    public boolean stEmpty() {
-      return false; // TODO!!! Your code here!
+      return this.lifo.isEmpty();
    }
 
    public void push (long a) {
-      // TODO!!! Your code here!
+      this.lifo.push(a);
    }
 
    public long pop() {
-      return 0; // TODO!!! Your code here!
-   } // pop
+      return this.lifo.pop();
+   }
 
    public void op (String s) {
-      // TODO!!!
+      if (s.equals("+")) {
+         this.lifo.push(this.lifo.pop() + this.lifo.pop());
+      } else if (s.equals("-")) {
+         long r1 = this.pop();
+         long r2 = this.pop();
+         this.lifo.push(r2 - r1);
+      } else if (s.equals("*")) {
+         this.lifo.push(this.lifo.pop() * this.lifo.pop());
+      } else if (s.equals("/")) {
+         long r1 = this.pop();
+         long r2 = this.pop();
+         this.lifo.push(r2 / r1);
+      } else if(s.equals(" ") | s.equals("\t")) {
+         return;
+      } else {
+         throw new RuntimeException("Invalid operation");
+      }
    }
   
    public long tos() {
-      return 0; // TODO!!! Your code here!
+      return this.lifo.getFirst();
    }
 
    @Override
    public boolean equals (Object o) {
-      return true; // TODO!!! Your code here!
+      return this.lifo.equals(((LongStack)o).lifo);
    }
 
    @Override
    public String toString() {
-      return null; // TODO!!! Your code here!
+      StringBuilder s = new StringBuilder();
+      Iterator<Long> i = this.lifo.descendingIterator();
+      while (i.hasNext()) {
+         s.append(i.next());
+         s.append(" ");
+      }
+      return s.toString();
    }
 
    public static long interpret (String pol) {
-      return 0; // TODO!!! Your code here!
+      LongStack ls = new LongStack();
+      for(int i = 0; i < pol.length(); i++) {
+         char c = pol.charAt(i);
+         char nc = ' ';
+         if (i+1 < pol.length()) {
+            nc = pol.charAt(i + 1);
+         }
+         if ((c == '-' & nc >= '0' & nc <= '9')| (c >= '0' & c <= '9')) {
+            StringBuilder buf = new StringBuilder();
+            buf.append(c);
+            i++;
+            for (; i < pol.length(); i++) {
+               c = pol.charAt(i);
+               if (c >= '0' & c <= '9') {
+                  buf.append(c);
+               } else {
+                  break;
+               }
+            }
+            ls.push(Long.parseLong(buf.toString()));
+         } else {
+            ls.op("" + c);
+         }
+      }
+      if (ls.lifo.size() == 1) {
+         return ls.pop();
+      } else {
+         throw new RuntimeException("Unbalanced polish notation");
+      }
+
    }
 
 }