fix: update some code

Author: Mayandev

leetcode刷题/note/No155_min-stack.md

@@ -31,15 +31,13 @@ js直接使用数组实现栈，最小值通过遍历查找，代码如下：
  */
 var MinStack = function() {
     this.stack = [];
-    this.min = 0;
 };
 
 /** 
  * @param {number} x
  * @return {void}
  */
 MinStack.prototype.push = function(x) {
-    if (this.stack[this.stack.length - 1] )
     this.stack.push(x);
 };