avc278 - 1.09 - Javascript (#64)

Added my solution to the Chapter 1 Problem 9 folder.
This is the final problem in chapter 1 tada

Author: avc278

JavaScript/chapter01/p09_string_rotation/avc278.js

@@ -0,0 +1,35 @@
+// String Rotation: Assume you have a method `isSubstring` which checks if one word is a substring of another. Given two
+// strings, s1 and s2, write code to check if s2 is a rotation of s1 using only one call to `isSubstring`.
+
+const assert = require("assert");
+
+/**
+ * Checks if one string is a rotation of another string
+ * @param  {string} s1 input string to check against
+ * @param  {string} s2 input string to check if is a rotation of s1
+ * @return {boolean}   whether s2 is a rotation of s1
+ *
+ * We can use the built-in method String.prototype.includes() to solve the problem for us,
+ * checking if s1s1 of size 2N, where N is the length of s1, contains the string s2, also of size N at this point.
+ *
+ * Runtime: O(N)
+ * Space:   O(1)
+ *
+ */
+const isSubstring = (s1, s2) => {
+  if (s1.length !== s2.length) return false;
+  return `${s1}${s1}`.includes(s2);
+};
+
+describe(module.filename, () => {
+  it("should return false when both input strings are not of the same length", () => {
+    const s1 = "hi";
+    const s2 = "hello";
+    assert.ok(!isSubstring(s1, s2));
+  });
+  it("should return true when s2 is a valid rotation of s1", () => {
+    const s1 = "waterbottle";
+    const s2 = "erbottlewat";
+    assert.ok(isSubstring(s1, s2));
+  });
+});