Merge pull request #166 from dunera/master

第9题-增加出队为空的判断

notes/剑指 offer 题解.md

@@ -393,11 +393,14 @@ public void push(int node) {
     in.push(node);
 }
 
-public int pop() {
+public int pop() throws Exception{
     if (out.isEmpty()) {
         while (!in.isEmpty()) {
             out.push(in.pop());
         }
+        if (out.isEmpty()){
+            throw new Exception("queue is empty");
+        }
     }
     return out.pop();
 }