update exercises

data/2.算法中阶/4.leetcode-栈与队列/6.155-最小栈/solution.md

@@ -21,24 +21,18 @@
 [null,null,null,null,-3,null,0,-2]
 
 <strong>解释：</strong>
+</pre>
 
+```
 MinStack minStack = new MinStack();
-
 minStack.push(-2);
-
 minStack.push(0);
-
 minStack.push(-3);
-
 minStack.getMin();   --&gt; 返回 -3.
-
 minStack.pop();
-
 minStack.top();      --&gt; 返回 0.
-
 minStack.getMin();   --&gt; 返回 -2.
-
-</pre>
+```
 
 <p>&nbsp;</p>