avc278 - 1.03 - Javascript (#58)

Added my solution to the Chapter 1 Problem 3 folder.

Author: avc278

JavaScript/chapter01/p03_urlify/avc278.js

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+// URLify: Write a method to replace all spaces in a string with '%20'. You may assume that the string has sufficient
+// space at the end to hold the additional characters, and that you are given the "true" length of the string.
+
+const assert = require("assert");
+
+/**
+ * URLifies an input string by replacing ' ' with '%20' and returning the modified string
+ * @param  {string} str        input string to modify
+ * @param  {int}    trueLength the "true" length of the str without trailing spaces
+ * @return {string}            output URLified string
+ *
+ * Since we cannot mutate a string in javascript, we need to use an external data structure, like an array.
+ * Creating a new data structure takes up O(N) space, where N is the length of the str.
+ * We can loop through the array from the end of the true length to the start, while keeping count of two indexes.
+ * If we encounter a space within the parameters of the word's true length, we can overwrite the element at that index
+ * and the previous two indices with "%20", and decrement the index by 3, all of which are O(1) operations.
+ * Alteranatively, we just overwrite the index of the str length counter with the index of the true length counter,
+ * and decrement the index by 1, which is also just an O(1) operation.
+ * When we're done traversing the array, we can simply return the joined array to return a string.
+ *
+ * Runtime: O(N)
+ * Space:   O(N)
+ *
+ */
+const urlify = (str, trueLength) => {
+  const strArr = str.split("");
+  let idx = str.length - 1;
+
+  for (let i = trueLength - 1; i >= 0; i--) {
+    if (strArr[i] === " ") {
+        strArr[idx] = "0";
+        strArr[idx - 1] = "2";
+        strArr[idx - 2] = "%";
+      idx -= 3;
+    } else {
+        strArr[idx] = strArr[i];
+      idx -= 1;
+    }
+  }
+  return strArr.join("");
+};
+
+describe(module.filename, () => {
+  it("should return the modified string with %20 in place of spaces", () => {
+    assert.equal(urlify("Mr John Smith    ", 13), "Mr%20John%20Smith");
+  });
+  it("should not modify a string with no spaces", () => {
+    assert.equal(urlify("techqueria", 10), "techqueria");
+  });
+});