add solution for 155 in js

Author: limbowandering

solution/155.Min Stack/Solution.js

@@ -0,0 +1,62 @@
+/**
+ * initialize your data structure here.
+ */
+const MinStack = function() {
+  this.arr = [];
+  this.help = [];  
+};
+
+/** 
+ * @param {number} x
+ * @return {void}
+ */
+MinStack.prototype.push = function(x) {
+    this.arr.push(x);
+    if(this.help.length === 0){
+      this.help.push(0);
+    }else{
+      let min = this.getMin();
+      if(x < min){
+        this.help.push(this.arr.length-1);
+      }
+    }
+};
+
+/**
+ * @return {void}
+ */
+MinStack.prototype.pop = function() {
+    if(this.arr.length === 0){
+      throw new Error('???');
+    }
+    if(this.arr.length - 1 === this.help[this.help.length - 1]){
+      this.help.pop();
+    }
+    this.arr.pop();
+};
+
+/**
+ * @return {number}
+ */
+MinStack.prototype.top = function() {
+    return this.arr[this.arr.length-1];
+};
+
+/**
+ * @return {number}
+ */
+MinStack.prototype.getMin = function() {
+    if(this.arr.length === 0){
+      throw new Error("???");
+    }
+    return this.arr[this.help[this.help.length-1]];
+};
+
+/** 
+ * Your MinStack object will be instantiated and called as such:
+ * var obj = Object.create(MinStack).createNew()
+ * obj.push(x)
+ * obj.pop()
+ * var param_3 = obj.top()
+ * var param_4 = obj.getMin()
+ */