feat: add js solution to lcci problem: No.08.01.Three Steps Problem (doocs#393)

Author: zhaocchen

lcci/08.01.Three Steps Problem/README.md

@@ -73,6 +73,23 @@ class Solution {
 }
 ```
 
+### **JavaScript**
+
+```js
+/**
+ * @param {number} n
+ * @return {number}
+ */
+ var waysToStep = function(n) {
+    if (n < 3) return n;
+    let a = 1, b = 2, c = 4;
+    for (let i = 3; i < n; i++) {
+        [a, b, c] = [b, c, (a + b + c) % 1000000007];
+    }
+    return c;
+};
+```
+
 ### **C++**
 
 ```cpp

lcci/08.01.Three Steps Problem/README_EN.md

@@ -67,6 +67,23 @@ class Solution {
 }
 ```
 
+### **JavaScript**
+
+```js
+/**
+ * @param {number} n
+ * @return {number}
+ */
+ var waysToStep = function(n) {
+    if (n < 3) return n;
+    let a = 1, b = 2, c = 4;
+    for (let i = 3; i < n; i++) {
+        [a, b, c] = [b, c, (a + b + c) % 1000000007];
+    }
+    return c;
+};
+```
+
 ### **C++**
 
 ```cpp

lcci/08.01.Three Steps Problem/Solution.js

@@ -0,0 +1,12 @@
+/**
+ * @param {number} n
+ * @return {number}
+ */
+ var waysToStep = function(n) {
+    if (n < 3) return n;
+    let a = 1, b = 2, c = 4;
+    for (let i = 3; i < n; i++) {
+        [a, b, c] = [b, c, (a + b + c) % 1000000007];
+    }
+    return c;
+};