feat: add question 155

Author: yinxin630

Diff for: 155.最小栈.js

@@ -0,0 +1,59 @@
+/*
+ * @lc app=leetcode.cn id=155 lang=javascript
+ *
+ * [155] 最小栈
+ * 
+ * 1. 用两个栈, A 栈存数据, B 栈存当前时刻的最小值
+ * 2. top 即是 A 栈顶, min 即是 B 栈顶
+ */
+
+// @lc code=start
+/**
+ * initialize your data structure here.
+ */
+var MinStack = function() {
+    this.stack = [];
+    this.min = [];
+};
+
+/** 
+ * @param {number} x
+ * @return {void}
+ */
+MinStack.prototype.push = function(x) {
+    this.min.push(this.stack.length === 0 ? x : Math.min(this.min[this.min.length - 1], x));
+    this.stack.push(x);
+};
+
+/**
+ * @return {void}
+ */
+MinStack.prototype.pop = function() {
+    this.stack.pop();
+    this.min.pop();
+};
+
+/**
+ * @return {number}
+ */
+MinStack.prototype.top = function() {
+    return this.stack[this.stack.length - 1];
+};
+
+/**
+ * @return {number}
+ */
+MinStack.prototype.getMin = function() {
+    return this.min[this.min.length - 1];
+};
+
+/** 
+ * Your MinStack object will be instantiated and called as such:
+ * var obj = new MinStack()
+ * obj.push(x)
+ * obj.pop()
+ * var param_3 = obj.top()
+ * var param_4 = obj.getMin()
+ */
+// @lc code=end
+